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Versions: 02 03 04 05 06 07 08 09 RFC 2408

IPSEC Working Group                   Douglas Maughan, Mark Schertler
INTERNET-DRAFT                            Mark Schneider, Jeff Turner
draft-ietf-ipsec-isakmp-09.txt, .ps                    March 10, 1998


    Internet Security Association and Key Management Protocol (ISAKMP)




                                 Abstract


     This memo describes a protocol utilizing security concepts
    necessary for establishing Security Associations (SA) and crypto-
    graphic keys in an Internet environment.  A Security Association
    protocol that negotiates, establishes, modifies and deletes
    Security Associations and their attributes is required for an
    evolving Internet, where there will be numerous security mecha-
    nisms and several options for each security mechanism.  The key
    management protocol must be robust in order to handle public key
    generation for the Internet community at large and private key
    requirements for those private networks with that requirement.
     The Internet Security Association and Key Management Protocol
    (ISAKMP) defines the procedures for authenticating a communicat-
    ing peer, creation and management of Security Associations, key
    generation techniques, and threat mitigation (e.g.  denial of
    service and replay attacks).  All of these are necessary to es-
    tablish and maintain secure communications (via IP Security Ser-
    vice or any other security protocol) in an Internet environment.



                           Status of this memo


This document is being submitted to the IETF Internet Protocol Security
(IPSEC) Working Group for consideration as a method for the establishment
and management of security associations and their appropriate security at-
tributes.  Additionally, this document proposes a method for key manage-
ment to support IPSEC and IPv6.  It is intended that a future version of
this draft be submitted to the IESG for publication as a Draft Standard
RFC. Comments are solicited and should be addressed to the authors and/or
the IPSEC working group mailing list at ipsec@tis.com.

This document is an Internet Draft.  Internet Drafts are working documents
of the Internet Engineering Task Force (IETF), its Areas, and its Working
Groups.  Note that other groups may also distribute working documents as
Internet Drafts.


INTERNET-DRAFT                    ISAKMP                    March 10, 1998

Internet Drafts are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other documents
at any time.  It is not appropriate to use Internet Drafts as reference
material or to cite them other than as ``working draft'' or ``work in
progress.''

To learn the current status of any Internet-Draft, please check the ``1id-
abstracts.txt'' listing contained in the Internet- Drafts Shadow Di-
rectories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

Distribution of this document is unlimited.









































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Contents

1 Introduction                                                           6
  1.1 Requirements Terminology  . . . . . . . . . . . . . . . . . . . .  7
  1.2 The Need for Negotiation  . . . . . . . . . . . . . . . . . . . .  7
  1.3 What can be Negotiated? . . . . . . . . . . . . . . . . . . . . . 7
  1.4 Security Associations and Management  . . . . . . . . . . . . . .  8
    1.4.1Security Associations and Registration . . . . . . . . . . . . 8
    1.4.2ISAKMP Requirements  . . . . . . . . . . . . . . . . . . . . .  9
  1.5 Authentication  . . . . . . . . . . . . . . . . . . . . . . . . .  9
    1.5.1Certificate Authorities  . . . . . . . . . . . . . . . . . . . 10
    1.5.2Entity Naming  . . . . . . . . . . . . . . . . . . . . . . . . 10
    1.5.3ISAKMP Requirements  . . . . . . . . . . . . . . . . . . . . . 11
  1.6 Public Key Cryptography . . . . . . . . . . . . . . . . . . . . . 12
    1.6.1Key Exchange Properties  . . . . . . . . . . . . . . . . . . . 12
    1.6.2ISAKMP Requirements  . . . . . . . . . . . . . . . . . . . . . 13
  1.7 ISAKMP Protection . . . . . . . . . . . . . . . . . . . . . . . . 13
    1.7.1Anti-Clogging (Denial of Service)  . . . . . . . . . . . . . . 13
    1.7.2Connection Hijacking . . . . . . . . . . . . . . . . . . . . . 14
    1.7.3Man-in-the-Middle Attacks  . . . . . . . . . . . . . . . . . . 14
  1.8 Multicast Communications  . . . . . . . . . . . . . . . . . . . . 14

2 Terminology and Concepts                                              15
  2.1 ISAKMP Terminology  . . . . . . . . . . . . . . . . . . . . . . . 15
  2.2 ISAKMP Placement  . . . . . . . . . . . . . . . . . . . . . . . . 17
  2.3 Negotiation Phases  . . . . . . . . . . . . . . . . . . . . . . . 18
  2.4 Identifying Security Associations . . . . . . . . . . . . . . . . 19
  2.5 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . 21
    2.5.1Transport Protocol . . . . . . . . . . . . . . . . . . . . . . 21
    2.5.2RESERVED Fields  . . . . . . . . . . . . . . . . . . . . . . . 21
    2.5.3Anti-Clogging Token (``Cookie'') Creation  . . . . . . . . . . 21
3 ISAKMP Payloads                                                       22
  3.1 ISAKMP Header Format  . . . . . . . . . . . . . . . . . . . . . . 22
  3.2 Generic Payload Header  . . . . . . . . . . . . . . . . . . . . . 26
  3.3 Data Attributes . . . . . . . . . . . . . . . . . . . . . . . . . 26
  3.4 Security Association Payload  . . . . . . . . . . . . . . . . . . 27
  3.5 Proposal Payload  . . . . . . . . . . . . . . . . . . . . . . . . 29
  3.6 Transform Payload . . . . . . . . . . . . . . . . . . . . . . . . 30
  3.7 Key Exchange Payload  . . . . . . . . . . . . . . . . . . . . . . 31
  3.8 Identification Payload  . . . . . . . . . . . . . . . . . . . . . 32
  3.9 Certificate Payload . . . . . . . . . . . . . . . . . . . . . . . 33
  3.10Certificate Request Payload . . . . . . . . . . . . . . . . . . . 35
  3.11Hash Payload  . . . . . . . . . . . . . . . . . . . . . . . . . . 36
  3.12Signature Payload . . . . . . . . . . . . . . . . . . . . . . . . 37
  3.13Nonce Payload . . . . . . . . . . . . . . . . . . . . . . . . . . 37
  3.14Notification Payload  . . . . . . . . . . . . . . . . . . . . . . 38
    3.14.1Notify Message Types . . . . . . . . . . . . . . . . . . . . . 40
  3.15Delete Payload  . . . . . . . . . . . . . . . . . . . . . . . . . 41
  3.16Vendor ID Payload . . . . . . . . . . . . . . . . . . . . . . . . 43

4 ISAKMP Exchanges                                                      45


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  4.1 ISAKMP Exchange Types . . . . . . . . . . . . . . . . . . . . . . 45
    4.1.1Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
  4.2 Security Association Establishment  . . . . . . . . . . . . . . . 46
    4.2.1Security Association Establishment Examples  . . . . . . . . . 48
  4.3 Security Association Modification . . . . . . . . . . . . . . . . 50
  4.4 Base Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . 51
  4.5 Identity Protection Exchange  . . . . . . . . . . . . . . . . . . 52
  4.6 Authentication Only Exchange  . . . . . . . . . . . . . . . . . . 53
  4.7 Aggressive Exchange . . . . . . . . . . . . . . . . . . . . . . . 54
  4.8 Informational Exchange  . . . . . . . . . . . . . . . . . . . . . 56

5 ISAKMP Payload Processing                                             56
  5.1 General Message Processing  . . . . . . . . . . . . . . . . . . . 57
  5.2 ISAKMP Header Processing  . . . . . . . . . . . . . . . . . . . . 57
  5.3 Generic Payload Header Processing . . . . . . . . . . . . . . . . 59
  5.4 Security Association Payload Processing . . . . . . . . . . . . . 60
  5.5 Proposal Payload Processing . . . . . . . . . . . . . . . . . . . 62
  5.6 Transform Payload Processing  . . . . . . . . . . . . . . . . . . 63
  5.7 Key Exchange Payload Processing . . . . . . . . . . . . . . . . . 64
  5.8 Identification Payload Processing . . . . . . . . . . . . . . . . 65
  5.9 Certificate Payload Processing  . . . . . . . . . . . . . . . . . 66
  5.10Certificate Request Payload Processing  . . . . . . . . . . . . . 67
  5.11Hash Payload Processing . . . . . . . . . . . . . . . . . . . . . 68
  5.12Signature Payload Processing  . . . . . . . . . . . . . . . . . . 69
  5.13Nonce Payload Processing  . . . . . . . . . . . . . . . . . . . . 70
  5.14Notification Payload Processing . . . . . . . . . . . . . . . . . 71
  5.15Delete Payload Processing . . . . . . . . . . . . . . . . . . . . 73
6 Conclusions                                                           75

A ISAKMP Security Association Attributes                                76
  A.1 Background/Rationale  . . . . . . . . . . . . . . . . . . . . . . 76
  A.2 Internet IP Security DOI Assigned Value . . . . . . . . . . . . . 76
  A.3 Supported Security Protocols  . . . . . . . . . . . . . . . . . . 76
  A.4 ISAKMP Identification Type Values . . . . . . . . . . . . . . . . 77
    A.4.1ID_IPV4_ADDR  . . . . . . . . . . . . . . . . . . . . . . . . . 77
    A.4.2ID_IPV4_ADDR_SUBNET . . . . . . . . . . . . . . . . . . . . . . 77
    A.4.3ID_IPV6_ADDR  . . . . . . . . . . . . . . . . . . . . . . . . . 77
    A.4.4ID_IPV6_ADDR_SUBNET . . . . . . . . . . . . . . . . . . . . . . 77
B Defining a new Domain of Interpretation                               78
  B.1 Situation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
  B.2 Security Policies . . . . . . . . . . . . . . . . . . . . . . . . 79
  B.3 Naming Schemes  . . . . . . . . . . . . . . . . . . . . . . . . . 79
  B.4 Syntax for Specifying Security Services . . . . . . . . . . . . . 79
  B.5 Payload Specification . . . . . . . . . . . . . . . . . . . . . . 79
  B.6 Defining new Exchange Types . . . . . . . . . . . . . . . . . . . 79








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List of Figures

  1   ISAKMP Relationships  . . . . . . . . . . . . . . . . . . . . . . 17
  2   ISAKMP Header Format  . . . . . . . . . . . . . . . . . . . . . . 23
  3   Generic Payload Header  . . . . . . . . . . . . . . . . . . . . . 26
  4   Data Attributes . . . . . . . . . . . . . . . . . . . . . . . . . 27
  5   Security Association Payload  . . . . . . . . . . . . . . . . . . 28
  6   Proposal Payload Format . . . . . . . . . . . . . . . . . . . . . 29
  7   Transform Payload Format  . . . . . . . . . . . . . . . . . . . . 30
  8   Key Exchange Payload Format . . . . . . . . . . . . . . . . . . . 32
  9   Identification Payload Format . . . . . . . . . . . . . . . . . . 33
  10  Certificate Payload Format  . . . . . . . . . . . . . . . . . . . 34
  11  Certificate Request Payload Format  . . . . . . . . . . . . . . . 35
  12  Hash Payload Format . . . . . . . . . . . . . . . . . . . . . . . 36
  13  Signature Payload Format  . . . . . . . . . . . . . . . . . . . . 37
  14  Nonce Payload Format  . . . . . . . . . . . . . . . . . . . . . . 38
  15  Notification Payload Format . . . . . . . . . . . . . . . . . . . 39
  16  Delete Payload Format . . . . . . . . . . . . . . . . . . . . . . 42
  17  Vendor ID Payload Format  . . . . . . . . . . . . . . . . . . . . 44


































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1 Introduction


This document describes an Internet Security Association and Key Manage-
ment Protocol (ISAKMP). ISAKMP combines the security concepts of authen-
tication, key management, and security associations to establish the re-
quired security for government, commercial, and private communications on
the Internet.

The Internet Security Association and Key Management Protocol (ISAKMP) de-
fines procedures and packet formats to establish, negotiate, modify and
delete Security Associations (SA). SAs contain all the information re-
quired for execution of various network security services, such as the
IP layer services (such as header authentication and payload encapsula-
tion), transport or application layer services, or self-protection of ne-
gotiation traffic.  ISAKMP defines payloads for exchanging key generation
and authentication data.  These formats provide a consistent framework for
transferring key and authentication data which is independent of the key
generation technique, encryption algorithm and authentication mechanism.

ISAKMP is distinct from key exchange protocols in order to cleanly sepa-
rate the details of security association management (and key management)
from the details of key exchange.  There may be many different key ex-
change protocols, each with different security properties.  However, a
common framework is required for agreeing to the format of SA attributes,
and for negotiating, modifying, and deleting SAs.  ISAKMP serves as this
common framework.

Separating the functionality into three parts adds complexity to the se-
curity analysis of a complete ISAKMP implementation.  However, the sep-
aration is critical for interoperability between systems with differing
security requirements, and should also simplify the analysis of further
evolution of a ISAKMP server.

ISAKMP is intended to support the negotiation of SAs for security proto-
cols at all layers of the network stack (e.g., IPSEC, TLS, TLSP, OSPF,
etc.).  By centralizing the management of the security associations,
ISAKMP reduces the amount of duplicated functionality within each security
protocol.  ISAKMP can also reduce connection setup time, by negotiating a
whole stack of services at once.

The remainder of section 1 establishes the motivation for security nego-
tiation and outlines the major components of ISAKMP, i.e.  Security As-
sociations and Management, Authentication, Public Key Cryptography, and
Miscellaneous items.  Section 2 presents the terminology and concepts as-
sociated with ISAKMP. Section 3 describes the different ISAKMP payload
formats.  Section 4 describes how the payloads of ISAKMP are composed to-
gether as exchange types to establish security associations and perform
key exchanges in an authenticated manner.  Additionally, security as-
sociation modification, deletion, and error notification are discussed.
Section 5 describes the processing of each payload within the context of


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ISAKMP exchanges, including error handling and associated actions.  The
appendices provide the attribute values necessary for ISAKMP and require-
ment for defining a new Domain of Interpretation (DOI) within ISAKMP.



1.1 Requirements Terminology


The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD
NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document,
are to be interpreted as described in [RFC-2119].


1.2 The Need for Negotiation


ISAKMP extends the assertion in [DOW92] that authentication and key ex-
changes must be combined for better security to include security associa-
tion exchanges.  The security services required for communications depends
on the individual network configurations and environments.  Organizations
are setting up Virtual Private Networks (VPN), also known as Intranets,
that will require one set of security functions for communications within
the VPN and possibly many different security functions for communications
outside the VPN to support geographically separate organizational compo-
nents, customers, suppliers, sub-contractors (with their own VPNs), gov-
ernment, and others.  Departments within large organizations may require a
number of security associations to separate and protect data (e.g.  per-
sonnel data, company proprietary data, medical) on internal networks and
other security associations to communicate within the same department.
Nomadic users wanting to ``phone home'' represent another set of secu-
rity requirements.  These requirements must be tempered with bandwidth
challenges.  Smaller groups of people may meet their security require-
ments by setting up ``Webs of Trust''.  ISAKMP exchanges provide these
assorted networking communities the ability to present peers with the se-
curity functionality that the user supports in an authenticated and pro-
tected manner for agreement upon a common set of security attributes, i.e.
an interoperable security association.


1.3 What can be Negotiated?


Security associations must support different encryption algorithms, au-
thentication mechanisms, and key establishment algorithms for other secu-
rity protocols, as well as IP Security.  Security associations must also
support host-oriented certificates for lower layer protocols and user-
oriented certificates for higher level protocols.  Algorithm and mecha-
nism independence is required in applications such as e-mail, remote lo-
gin, and file transfer, as well as in session oriented protocols, routing
protocols, and link layer protocols.  ISAKMP provides a common security


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association and key establishment protocol for this wide range of security
protocols, applications, security requirements, and network environments.

ISAKMP is not bound to any specific cryptographic algorithm, key gener-
ation technique, or security mechanism.  This flexibility is beneficial
for a number of reasons.  First, it supports the dynamic communications
environment described above.  Second, the independence from specific secu-
rity mechanisms and algorithms provides a forward migration path to better
mechanisms and algorithms.  When improved security mechanisms are devel-
oped or new attacks against current encryption algorithms, authentica-
tion mechanisms and key exchanges are discovered, ISAKMP will allow the
updating of the algorithms and mechanisms without having to develop a com-
pletely new KMP or patch the current one.

ISAKMP has basic requirements for its authentication and key exchange com-
ponents.  These requirements guard against denial of service, replay / re-
flection, man-in-the-middle, and connection hijacking attacks.  This is
important because these are the types of attacks that are targeted against
protocols.  Complete Security Association (SA) support, which provides
mechanism and algorithm independence, and protection from protocol threats
are the strengths of ISAKMP.



1.4 Security Associations and Management


A Security Association (SA) is a relationship between two or more entities
that describes how the entities will utilize security services to communi-
cate securely.  This relationship is represented by a set of information
that can be considered a contract between the entities.  The information
must be agreed upon and shared between all the entities.  Sometimes the
information alone is referred to as an SA, but this is just a physical in-
stantiation of the existing relationship.  The existence of this relation-
ship, represented by the information, is what provides the agreed upon se-
curity information needed by entities to securely interoperate.  All enti-
ties must adhere to the SA for secure communications to be possible.  When
accessing SA attributes, entities use a pointer or identifier refered to
as the Security Parameter Index (SPI). [RFC-1825] provides details on IP
Security Associations (SA) and Security Parameter Index (SPI) definitions.


1.4.1 Security Associations and Registration


The SA attributes required and recommended for the IP Security (AH, ESP)
are defined in [RFC-1825].  The attributes specified for an IP Security SA
include, but are not limited to, authentication mechanism, cryptographic
algorithm, algorithm mode, key length, and Initialization Vector (IV).
Other protocols that provide algorithm and mechanism independent secu-
rity MUST define their requirements for SA attributes.  The separation of
ISAKMP from a specific SA definition is important to ensure ISAKMP can es-

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tablish SAs for all possible security protocols and applications.

NOTE: See [IPDOI] for a discussion of SA attributes that should be consid-
ered when defining a security protocol or application.

In order to facilitate easy identification of specific attributes (e.g.
a specific encryption algorithm) among different network entites the at-
tributes must be assigned identifiers and these identifiers must be reg-
istered by a central authority.  The Internet Assigned Numbers Authority
(IANA) provides this function for the Internet.


1.4.2 ISAKMP Requirements


Security Association (SA) establishment MUST be part of the key manage-
ment protocol defined for IP based networks.  The SA concept is required
to support security protocols in a diverse and dynamic networking envi-
ronment.  Just as authentication and key exchange must be linked to pro-
vide assurance that the key is established with the authenticated party
[DOW92], SA establishment must be linked with the authentication and the
key exchange protocol.

ISAKMP provides the protocol exchanges to establish a security association
between negotiating entities followed by the establishment of a security
association by these negotiating entities in behalf of some protocol (e.g.
ESP/AH). First, an initial protocol exchange allows a basic set of secu-
rity attributes to be agreed upon.  This basic set provides protection for
subsequent ISAKMP exchanges.  It also indicates the authentication method
and key exchange that will be performed as part of the ISAKMP protocol.
If a basic set of security attributes is already in place between the ne-
gotiating server entities, the initial ISAKMP exchange may be skipped and
the establishment of a security association can be done directly.  After
the basic set of security attributes has been agreed upon, initial iden-
tity authenticated, and required keys generated, the established SA can
be used for subsequent communications by the entity that invoked ISAKMP.
The basic set of SA attributes that MUST be implemented to provide ISAKMP
interoperability are defined in Appendix A.



1.5 Authentication


A very important step in establishing secure network communications is au-
thentication of the entity at the other end of the communication.  Many
authentication mechanisms are available.  Authentication mechanisms fall
into two catagories of strength - weak and strong.  Sending cleartext keys
or other unprotected authenticating information over a network is weak,
due to the threat of reading them with a network sniffer.  Additionally,
sending one-way hashed poorly-chosen keys with low entropy is also weak,
due to the threat of brute-force guessing attacks on the sniffed mes-

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sages.  While passwords can be used for establishing identity, they are
not considered in this context because of recent statements from the In-
ternet Architecture Board [IAB].  Digital signatures, such as the Digital
Signature Standard (DSS) and the Rivest-Shamir-Adleman (RSA) signature,
are public key based strong authentication mechanisms.  When using pub-
lic key digital signatures each entity requires a public key and a pri-
vate key.  Certificates are an essential part of a digital signature au-
thentication mechanism.  Certificates bind a specific entity's identity
(be it host, network, user, or application) to its public keys and pos-
sibly other security-related information such as privileges, clearances,
and compartments.  Authentication based on digital signatures requires a
trusted third party or certificate authority to create, sign and properly
distribute certificates.  For more detailed information on digital signa-
tures, such as DSS and RSA, and certificates see [Schneier].


1.5.1 Certificate Authorities


Certificates require an infrastructure for generation, verification, re-
vocation, management and distribution.  The Internet Policy Registration
Authority (IPRA) [RFC-1422] has been established to direct this infras-
tructure for the IETF. The IPRA certifies Policy Certification Authori-
ties (PCA). PCAs control Certificate Authorities (CA) which certify users
and subordinate entities.  Current certificate related work includes the
Domain Name System (DNS) Security Extensions [DNSSEC] which will provide
signed entity keys in the DNS. The Public Key Infrastucture (PKIX) working
group is specifying an Internet profile for X.509 certificates.  There is
also work going on in industry to develop X.500 Directory Services which
would provide X.509 certificates to users.  The U.S. Post Office is devel-
oping a (CA) hierarchy.  The NIST Public Key Infrastructure Working Group
has also been doing work in this area.  The DOD Multi Level Information
System Security Initiative (MISSI) program has begun deploying a certifi-
cate infrastructure for the U.S. Government.  Alternatively, if no infras-
tructure exists, the PGP Web of Trust certificates can be used to provide
user authentication and privacy in a community of users who know and trust
each other.


1.5.2 Entity Naming


An entity's name is its identity and is bound to its public keys in cer-
tificates.  The CA MUST define the naming semantics for the certificates
it issues.  See the UNINETT PCA Policy Statements [Berge] for an example
of how a CA defines its naming policy.  When the certificate is verified,
the name is verified and that name will have meaning within the realm of
that CA. An example is the DNS security extensions which make DNS servers
CAs for the zones and nodes they serve.  Resource records are provided for
public keys and signatures on those keys.  The names associated with the
keys are IP addresses and domain names which have meaning to entities ac-


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cessing the DNS for this information.  A Web of Trust is another example.
When webs of trust are set up, names are bound with the public keys.  In
PGP the name is usually the entity's e-mail address which has meaning to
those, and only those, who understand e-mail.  Another web of trust could
use an entirely different naming scheme.


1.5.3 ISAKMP Requirements


Strong authentication MUST be provided on ISAKMP exchanges.  Without being
able to authenticate the entity at the other end, the Security Association
(SA) and session key established are suspect.  Without authentication you
are unable to trust an entity's identification, which makes access control
questionable.  While encryption (e.g.  ESP) and integrity (e.g.  AH) will
protect subsequent communications from passive eavesdroppers, without au-
thentication it is possible that the SA and key may have been established
with an adversary who performed an active man-in-the-middle attack and is
now stealing all your personal data.

A digital signature algorithm MUST be used within ISAKMP's authentication
component.  However, ISAKMP does not mandate a specific signature algo-
rithm or certificate authority (CA). ISAKMP allows an entity initiating
communications to indicate which CAs it supports.  After selection of a
CA, the protocol provides the messages required to support the actual au-
thentication exchange.  The protocol provides a facility for identifica-
tion of different certificate authorities, certificate types (e.g.  X.509,
PKCS #7, PGP, DNS SIG and KEY records), and the exchange of the certifi-
cates identified.

ISAKMP utilizes digital signatures, based on public key cryptography, for
authentication.  There are other strong authentication systems available,
which could be specified as additional optional authentication mechanisms
for ISAKMP. Some of these authentication systems rely on a trusted third
party called a key distribution center (KDC) to distribute secret session
keys.  An example is Kerberos, where the trusted third party is the Ker-
beros server, which holds secret keys for all clients and servers within
its network domain.  A client's proof that it holds its secret key pro-
vides authenticaton to a server.

The ISAKMP specification does not specify the protocol for communicating
with the trusted third parties (TTP) or certificate directory services.
These protocols are defined by the TTP and directory service themselves
and are outside the scope of this specification.  The use of these addi-
tional services and protocols will be described in a Key Exchange specific
document.







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1.6 Public Key Cryptography


Public key cryptography is the most flexible, scalable, and efficient way
for users to obtain the shared secrets and session keys needed to support
the large number of ways Internet users will interoperate.  Many key gen-
eration algorithms, that have different properties, are available to users
(see [DOW92], [ANSI], and [Oakley]).  Properties of key exchange protocols
include the key establishment method, authentication, symmetry, perfect
forward secrecy, and back traffic protection.

NOTE: Cryptographic keys can protect information for a considerable length
of time.  However, this is based on the assumption that keys used for pro-
tection of communications are destroyed after use and not kept for any
reason.


1.6.1 Key Exchange Properties


Key Establishment (Key Generation / Key Transport): The two common methods
of using public key cryptography for key establishment are key transport
and key generation.  An example of key transport is the use of the RSA al-
gorithm to encrypt a randomly generated session key (for encrypting subse-
quent communications) with the recipient's public key.  The encrypted ran-
dom key is then sent to the recipient, who decrypts it using his private
key.  At this point both sides have the same session key, however it was
created based on input from only one side of the communications.  The ben-
efit of the key transport method is that it has less computational over-
head than the following method.  The Diffie-Hellman (D-H) algorithm il-
lustrates key generation using public key cryptography.  The D-H algorithm
is begun by two users exchanging public information.  Each user then math-
ematically combines the other's public information along with their own
secret information to compute a shared secret value.  This secret value
can be used as a session key or as a key encryption key for encrypting a
randomly generated session key.  This method generates a session key based
on public and secret information held by both users.  The benefit of the
D-H algorithm is that the key used for encrypting messages is based on
information held by both users and the independence of keys from one key
exchange to another provides perfect forward secrecy.  Detailed descrip-
tions of these algorithms can be found in [Schneier].  There are a number
of variations on these two key generation schemes and these variations do
not necessarily interoperate.


Key Exchange Authentication: Key exchanges may be authenticated during the
protocol or after protocol completion.  Authentication of the key exchange
during the protocol is provided when each party provides proof it has the
secret session key before the end of the protocol.  Proof can be provided
by encrypting known data in the secret session key during the protocol ex-
change.  Authentication after the protocol must occur in subsequent commu-


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nications.  Authentication during the protocol is preferred so subsequent
communications are not initiated if the secret session key is not estab-
lished with the desired party.


Key Exchange Symmetry: A key exchange provides symmetry if either party
can initiate the exchange and exchanged messages can cross in transit
without affecting the key that is generated.  This is desirable so that
computation of the keys does not require either party to know who initi-
ated the exchange.  While key exchange symmetry is desirable, symmetry in
the entire key management protocol may provide a vulnerablity to reflec-
tion attacks.


Perfect Forward Secrecy: As described in [DOW92], an authenticated key ex-
change protocol provides perfect forward secrecy if disclosure of long-
term secret keying material does not compromise the secrecy of the ex-
changed keys from previous communications.  The property of perfect for-
ward secrecy does not apply to key exchange without authentication.


1.6.2 ISAKMP Requirements


An authenticated key exchange MUST be supported by ISAKMP. Users SHOULD
choose additional key establishment algorithms based on their require-
ments.  ISAKMP does not specify a specific key exchange.  However, [IKE]
describes a proposal for using the Oakley key exchange [Oakley] in con-
junction with ISAKMP. Requirements that should be evaluated when choosing
a key establishment algorithm include establishment method (generation vs.
transport), perfect forward secrecy, computational overhead, key escrow,
and key strength.  Based on user requirements, ISAKMP allows an entity
initiating communications to indicate which key exchanges it supports.
After selection of a key exchange, the protocol provides the messages re-
quired to support the actual key establishment.



1.7 ISAKMP Protection


1.7.1 Anti-Clogging (Denial of Service)


Of the numerous security services available, protection against denial
of service always seems to be one of the most difficult to address.  A
``cookie'' or anti-clogging token (ACT) is aimed at protecting the com-
puting resources from attack without spending excessive CPU resources to
determine its authenticity.  An exchange prior to CPU-intensive public key
operations can thwart some denial of service attempts (e.g.  simple flood-
ing with bogus IP source addresses).  Absolute protection against denial
of service is impossible, but this anti-clogging token provides a tech-

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nique for making it easier to handle.  The use of an anti-clogging token
was introduced by Karn and Simpson in [Karn].

It should be noted that in the exchanges shown in section 4, the anti-
clogging mechanism should be used in conjuction with a garbage-state col-
lection mechanism; an attacker can still flood a server using packets with
bogus IP addresses and cause state to be created.  Such aggressive memory
management techniques SHOULD be employed by protocols using ISAKMP that
do not go through an initial, anti-clogging only phase, as was done in
[Karn].


1.7.2 Connection Hijacking


ISAKMP prevents connection hijacking by linking the authentication, key
exchange and security association exchanges.  This linking prevents an
attacker from allowing the authentication to complete and then jumping
in and impersonating one entity to the other during the key and security
association exchanges.


1.7.3 Man-in-the-Middle Attacks


Man-in-the-Middle attacks include interception, insertion, deletion, and
modification of messages, reflecting messages back at the sender, re-
playing old messages and redirecting messages.  ISAKMP features prevent
these types of attacks from being successful.  The linking of the ISAKMP
exchanges prevents the insertion of messages in the protocol exchange.
The ISAKMP protocol state machine is defined so deleted messages will not
cause a partial SA to be created, the state machine will clear all state
and return to idle.  The state machine also prevents reflection of a mes-
sage from causing harm.  The requirement for a new cookie with time vari-
ant material for each new SA establishment prevents attacks that involve
replaying old messages.  The ISAKMP strong authentication requirement pre-
vents an SA from being established with anyone other than the intended
party.  Messages may be redirected to a different destination or modified
but this will be detected and an SA will not be established.  The ISAKMP
specification defines where abnormal processing has occurred and recom-
mends notifying the appropriate party of this abnormality.



1.8 Multicast Communications


It is expected that multicast communications will require the same secu-
rity services as unicast communications and may introduce the need for
additional security services.  The issues of distributing SPIs for mul-
ticast traffic are presented in [RFC-1825].  Multicast security issues are
also discussed in [RFC-1949] and [BC].  A future extension to ISAKMP will

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support multicast key distribution.  For an introduction to the issues re-
lated to multicast security, consult the Internet Drafts, [RFC-2094] and
[RFC-2093], describing Sparta's research in this area.



2 Terminology and Concepts


2.1 ISAKMP Terminology


Security Protocol: A Security Protocol consists of an entity at a single
point in the network stack, performing a security service for network com-
munication.  For example, IPSEC ESP and IPSEC AH are two different secu-
rity protocols.  TLS is another example.  Security Protocols may perform
more than one service, for example providing integrity and confidentiality
in one module.


Protection Suite: A protection suite is a list of the security services
that must be applied by various security protocols.  For example, a pro-
tection suite may consist of DES encryption in IP ESP, and keyed MD5 in IP
AH. All of the protections in a suite must be treated as a single unit.
This is necessary because security services in different security pro-
tocols can have subtle interactions, and the effects of a suite must be
analyzed and verified as a whole.


Security Association (SA): A Security Association is a security-protocol-
specific set of parameters that completely defines the services and mech-
anisms necessary to protect traffic at that security protocol location.
These parameters can include algorithm identifiers, modes, cryptographic
keys, etc.  The SA is referred to by its associated security protocol (for
example, ``ISAKMP SA'', ``ESP SA'', ``TLS SA'').


ISAKMP SA: An SA used by the ISAKMP servers to protect their own traffic.
Sections 2.3 and 2.4 provide more details about ISAKMP SAs.


Security Parameter Index (SPI): An identifier for a Security Assocation,
relative to some security protocol.  Each security protocol has its own
``SPI-space''.  A (security protocol, SPI) pair may uniquely identify an
SA. The uniqueness of the SPI is implementation dependent, but could be
based per system, per protocol, or other options.  Depending on the DOI,
additional information (e.g.  host address) may be necessary to identify
an SA. The DOI will also determine which SPIs (i.e.  initiator's or re-
sponder's) are sent during communication.




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Domain of Interpretation: A Domain of Interpretation (DOI) defines payload
formats, exchange types, and conventions for naming security-relevant in-
formation such as security policies or cryptographic algorithms and modes.
A Domain of Interpretation (DOI) identifier is used to interpret the pay-
loads of ISAKMP payloads.  A system SHOULD support multiple Domains of In-
terpretation simultaneously.  The concept of a DOI is based on previous
work by the TSIG CIPSO Working Group, but extends beyond security label
interpretation to include naming and interpretation of security services.
A DOI defines:



 o  A ``situation'':  the set of information that will be used to
    determine the required security services.

 o  The set of security policies that must, and may, be supported.

 o  A syntax for the specification of proposed security services.

 o  A scheme for naming security-relevant information, including
    encryption algorithms, key exchange algorithms, security policy
    attributes, and certificate authorities.

 o  The specific formats of the various payload contents.

 o  Additional exchange types, if required.


The rules for the IETF IP Security DOI are presented in [IPDOI].  Speci-
fications of the rules for customized DOIs will be presented in separate
documents.


Situation: A situation contains all of the security-relevant information
that a system considers necessary to decide the security services required
to protect the session being negotiated.  The situation may include ad-
dresses, security classifications, modes of operation (normal vs.  emer-
gency), etc.


Proposal: A proposal is a list, in decreasing order of preference, of the
protection suites that a system considers acceptable to protect traffic
under a given situation.


Payload: ISAKMP defines several types of payloads, which are used to
transfer information such as security association data, or key exchange
data, in DOI-defined formats.  A payload consists of a generic payload
header and a string of octects that is opaque to ISAKMP. ISAKMP uses DOI-
specific functionality to synthesize and interpret these payloads.  Mul-
tiple payloads can be sent in a single ISAKMP message.  See section 3 for
more details on the payload types, and [IPDOI] for the formats of the IETF

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IP Security DOI payloads.


Exchange Type: An exchange type is a specification of the number of mes-
sages in an ISAKMP exchange, and the payload types that are contained in
each of those messages.  Each exchange type is designed to provide a par-
ticular set of security services, such as anonymity of the participants,
perfect forward secrecy of the keying material, authentication of the par-
ticipants, etc.  Section 4.1 defines the default set of ISAKMP exchange
types.  Other exchange types can be added to support additional key ex-
changes, if required.



2.2 ISAKMP Placement


Figure 1 is a high level view of the placement of ISAKMP within a system
context in a network architecture.  An important part of negotiating secu-
rity services is to consider the entire ``stack'' of individual SAs as a
unit.  This is referred to as a ``protection suite''.


     +------------+        +--------+                +--------------+
     !     DOI    !        !        !                !  Application !
     ! Definition ! <----> ! ISAKMP !                !    Process   !
     +------------+    --> !        !                !--------------!
    +--------------+   !   +--------+                ! Appl Protocol!
    ! Key Exchange !   !     ^  ^                    +--------------+
    !  Definition  !<--      !  !                           ^
    +--------------+         !  !                           !
                             !  !                           !
            !----------------!  !                           !
            v                   !                           !
        +-------+               v                           v
        !  API  !        +---------------------------------------------+
        +-------+        !                Socket Layer                 !
            !            !---------------------------------------------!
            v            !        Transport Protocol (TCP / UDP)       !
     +----------+        !---------------------------------------------!
     ! Security ! <----> !                     IP                      !
     ! Protocol !        !---------------------------------------------!
     +----------+        !             Link Layer Protocol             !
                         +---------------------------------------------+



                     Figure 1:  ISAKMP Relationships





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2.3 Negotiation Phases


ISAKMP offers two ``phases'' of negotiation.  In the first phase, two en-
tities (e.g.  ISAKMP servers) agree on how to protect further negotiation
traffic between themselves, establishing an ISAKMP SA. This ISAKMP SA is
then used to protect the negotiations for the Protocol SA being requested.
Two entities (e.g.  ISAKMP servers) can negotiate (and have active) multi-
ple ISAKMP SAs.

The second phase of negotiation is used to establish security associations
for other security protocols.  This second phase can be used to estab-
lish many security associations.  The security associations established
by ISAKMP during this phase can be used by a security protocol to protect
many message/data exchanges.

While the two-phased approach has a higher start-up cost for most simple
scenarios, there are several reasons that it is beneficial for most cases.

First, entities (e.g.  ISAKMP servers) can amortize the cost of the first
phase across several second phase negotiations.  This allows multiple SAs
to be established between peers over time without having to start over for
each communication.

Second, security services negotiated during the first phase provide secu-
rity properties for the second phase.  For example, after the first phase
of negotiation, the encryption provided by the ISAKMP SA can provide iden-
tity protection, potentially allowing the use of simpler second-phase ex-
changes.  On the other hand, if the channel established during the first
phase is not adequate to protect identities, then the second phase must
negotiate adequate security mechanisms.

Third, having an ISAKMP SA in place considerably reduces the cost of
ISAKMP management activity - without the ``trusted path'' that an ISAKMP
SA gives you, the entities (e.g.  ISAKMP servers) would have to go through
a complete re-authentication for each error notification or deletion of an
SA.

Negotiation during each phase is accomplished using ISAKMP-defined ex-
changes (see section 4) or exchanges defined for a key exchange within a
DOI.

Note that security services may be applied differently in each negotiation
phase.  For example, different parties are being authenticated during each
of the phases of negotiation.  During the first phase, the parties being
authenticated may be the ISAKMP servers/hosts, while during the second
phase, users or application level programs are being authenticated.






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2.4 Identifying Security Associations


While bootstrapping secure channels between systems, ISAKMP cannot assume
the existence of security services, and must provide some protections for
itself.  Therefore, ISAKMP considers an ISAKMP Security Association to
be different than other types, and manages ISAKMP SAs itself, in their
own name space.  ISAKMP uses the two cookie fields in the ISAKMP header
to identify ISAKMP SAs.  The Message ID in the ISAKMP Header and the SPI
field in the Proposal payload are used during SA establishment to identify
the SA for other security protocols.  The interpretation of these four
fields is dependent on the operation taking place.

The following table shows the presence or absence of several fields during
SA establishment.  The following fields are necessary for various opera-
tions associated with SA establishment:  cookies in the ISAKMP header, the
ISAKMP Header Message ID field, and the SPI field in the Proposal payload.
An 'X' in the column means the value MUST be present.  An 'NA' in the col-
umn means a value in the column is Not Applicable to the operation.




__#_____________Operation____________I-Cookie__R-Cookie__Message_ID__SPI_
 (1)  Start ISAKMP SA negotiation    X         0         0           0
 (2)  Respond ISAKMP SA negotiation  X         X         0           0
 (3)  Init other SA negotiation      X         X         X           X
 (4)  Respond other SA negotiation   X         X         X           X
 (5)  Other (KE, ID, etc.)           X         X         X/0         NA
 (6)  Security Protocol (ESP, AH)    NA        NA        NA          X


In the first line (1) of the table, the initiator includes the Initiator
Cookie field in the ISAKMP Header, using the procedures outlined in sec-
tions 2.5.3 and 3.1.

In the second line (2) of the table, the responder includes the Initia-
tor and Responder Cookie fields in the ISAKMP Header, using the procedures
outlined in sections 2.5.3 and 3.1.  Additional messages may be exchanged
between ISAKMP peers, depending on the ISAKMP exchange type used during
the phase 1 negotiation.  Once the phase 1 exchange is completed, the Ini-
tiator and Responder cookies are included in the ISAKMP Header of all sub-
sequent communications between the ISAKMP peers.

During phase 1 negotiations, the initiator and responder cookies deter-
mine the ISAKMP SA. Therefore, the SPI field in the Proposal payload is
redundant and MAY be set to 0 or it MAY contain the transmitting entity's
cookie.

In the third line (3) of the table, the initiator associates a Message ID
with the Protocols contained in the SA Proposal.  This Message ID and the
initiator's SPI(s) to be associated with each protocol in the Proposal are

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sent to the responder.  The SPI(s) will be used by the security protocols
once the phase 2 negotiation is completed.

In the fourth line (4) of the table, the responder includes the same Mes-
sage ID and the responder's SPI(s) to be associated with each protocol in
the accepted Proposal.  This information is returned to the initiator.

In the fifth line (5) of the table, the initiator and responder use the
Message ID field in the ISAKMP Header to keep track of the in-progress
protocol negotiation.  This is only applicable for a phase 2 exchange and
the value SHOULD be 0 for a phase 1 exchange because the combined cook-
ies identify the ISAKMP SA. The SPI field in the Proposal payload is not
applicable because the Proposal payload is only used during the SA negoti-
ation message exchange (steps 3 and 4).

In the sixth line (6) of the table, the phase 2 negotiation is complete.
The security protocols use the SPI(s) to determine which security services
and mechanisms to apply to the communication between them.  The SPI value
shown in the sixth line (6) is not the SPI field in the Proposal payload,
but the SPI field contained within the security protocol header.

During the SA establishment, a SPI MUST be generated.  ISAKMP is designed
to handle variable sized SPIs.  This is accomplished by using the SPI Size
field within the Proposal payload during SA establishment.  Handling of
SPIs will be outlined by the DOI specification (e.g.  [IPDOI]).

When a security association (SA) is initially established, one side as-
sumes the role of initiator and the other the role of responder.  Once the
SA is established, both the original initiator and responder can initiate
a phase 2 negotiation with the peer entity.  Thus, ISAKMP SAs are bidirec-
tional in nature.

Additionally, ISAKMP allows both initiator and responder to have some con-
trol during the negotiation process.  While ISAKMP is designed to allow an
SA negotiation that includes multiple proposals, the initiator can main-
tain some control by only making one proposal in accordance with the ini-
tiator's local security policy.  Once the initiator sends a proposal con-
taining more than one proposal (which are sent in decreasing preference
order), the initiator relinquishes control to the responder.  Once the re-
sponder is controlling the SA establishment, the responder can make its
policy take precedence over the initiator within the context of the multi-
ple options offered by the initiator.  This is accomplished by selecting
the proposal best suited for the responder's local security policy and re-
turning this selection to the initiator.









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2.5 Miscellaneous


2.5.1 Transport Protocol


ISAKMP can be implemented over any transport protocol or over IP itself.
Implementations MUST include send and receive capability for ISAKMP us-
ing the User Datagram Protocol (UDP) on port 500.  UDP Port 500 has been
assigned to ISAKMP by the Internet Assigned Numbered Authority (IANA). Im-
plementations MAY additionally support ISAKMP over other transport proto-
cols or over IP itself.


2.5.2 RESERVED Fields


The existence of RESERVED fields within ISAKMP payloads are used strictly
to preserve byte alignment.  All RESERVED fields in the ISAKMP protocol
MUST be set to zero (0) when a packet is issued.  The receiver SHOULD
check the RESERVED fields for a zero (0) value and discard the packet if
other values are found.


2.5.3 Anti-Clogging Token (``Cookie'') Creation


The details of cookie generation are implementation dependent, but MUST
satisfy these basic requirements (originally stated by Phil Karn in
[Karn]):



   1.    The cookie must depend on the specific parties.  This prevents
         an attacker from obtaining a cookie using a real IP address and
         UDP port, and then using it to swamp the victim with Diffie-
         Hellman requests from randomly chosen IP addresses or ports.

   2.    It must not be possible for anyone other than the issuing
         entity to generate cookies that will be accepted by that
         entity.  This implies that the issuing entity must use local
         secret information in the generation and subsequent
         verification of a cookie.  It must not be possible to deduce
         this secret information from any particular cookie.

   3.    The cookie generation function must be fast to thwart attacks
         intended to sabotage CPU resources.


Karn's suggested method for creating the cookie is to perform a fast hash
(e.g.  MD5) over the IP Source and Destination Address, the UDP Source and
Destination Ports and a locally generated secret random value.  ISAKMP re-

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quires that the cookie be unique for each SA establishment to help pre-
vent replay attacks, therefore, the date and time MUST be added to the in-
formation hashed.  The generated cookies are placed in the ISAKMP Header
(described in section 3.1) Initiator and Responder cookie fields.  These
fields are 8 octets in length, thus, requiring a generated cookie to be 8
octets.  Notify and Delete messages (see sections 3.14, 3.15, and 4.8) are
uni-directional transmissions and are done under the protection of an ex-
isting ISAKMP SA, thus, not requiring the generation of a new cookie.  One
exception to this is the transmission of a Notify message during a Phase
1 exchange, prior to completing the establishment of an SA. Sections 3.14
and 4.8 provide additional details.



3 ISAKMP Payloads


ISAKMP payloads provide modular building blocks for constructing ISAKMP
messages.  The presence and ordering of payloads in ISAKMP is defined by
and dependent upon the Exchange Type Field located in the ISAKMP Header
(see Figure 2).  The ISAKMP payload types are discussed in sections 3.4
through 3.15.  The descriptions of the ISAKMP payloads, messages, and ex-
changes (see Section 4) are shown using network octet ordering.  Addition-
ally, all ISAKMP messages MUST be aligned at 4-octet multiples.


3.1 ISAKMP Header Format


An ISAKMP message has a fixed header format, shown in Figure 2, followed
by a variable number of payloads.  A fixed header simplifies parsing, pro-
viding the benefit of protocol parsing software that is less complex and
easier to implement.  The fixed header contains the information required
by the protocol to maintain state, process payloads and possibly prevent
denial of service or replay attacks.

The ISAKMP Header fields are defined as follows:


 o  Initiator Cookie (8 octets) - Cookie of entity that initiated SA
    establishment, SA notification, or SA deletion.

 o  Responder Cookie (8 octets) - Cookie of entity that is responding to










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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                          Initiator                            !
        !                            Cookie                             !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                          Responder                            !
        !                            Cookie                             !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Next Payload ! MjVer ! MnVer ! Exchange Type !     Flags     !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                          Message ID                           !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                            Length                             !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 2:  ISAKMP Header Format


    an SA establishment request, SA notification, or SA deletion.

 o  Next Payload (1 octet) - Indicates the type of the first payload in
    the message.  The format for each payload is defined in sections 3.4
    through 3.16.  The processing for the payloads is defined in section
    5.


                    _____Next_Payload_Type_______Value____
                     NONE                          0
                     Security Association (SA)      1
                     Proposal (P)                   2
                     Transform (T)                  3
                     Key Exchange (KE)              4
                     Identification (ID)            5
                     Certificate (CERT)             6
                     Certificate Request (CR)       7
                     Hash (HASH)                    8
                     Signature (SIG)                9
                     Nonce (NONCE)                 10
                     Notification (N)              11
                     Delete (D)                    12
                     Vendor ID (VID)               13
                     RESERVED                   14 - 127
                     Private USE               128 - 255


 o  Major Version (4 bits) - indicates the major version of the ISAKMP
    protocol in use.  Implementations based on this version of the ISAKMP
    Internet-Draft MUST set the Major Version to 1.  Implementations


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    based on previous versions of ISAKMP Internet-Drafts MUST set the
    Major Version to 0.  Implementations SHOULD never accept packets with
    a major version number larger than its own.

 o  Minor Version (4 bits) - indicates the minor version of the ISAKMP
    protocol in use.  Implementations based on this version of the ISAKMP
    Internet-Draft MUST set the Minor Version to 0.  Implementations
    based on previous versions of ISAKMP Internet-Drafts MUST set the
    Minor Version to 1.  Implementations SHOULD never accept packets with
    a minor version number larger than its own, given the major version
    numbers are identical.

 o  Exchange Type (1 octet) - indicates the type of exchange being used.
    This dictates the message and payload orderings in the ISAKMP
    exchanges.



                       ____Exchange_Type______Value___
                        NONE                    0
                        Base                    1
                        Identity Protection     2
                        Authentication Only     3
                        Aggressive              4
                        Informational           5
                        ISAKMP Future Use     6 - 31
                        DOI Specific Use     32 - 255


 o  Flags (1 octet) - indicates specific options that are set for the
    ISAKMP exchange.  The flags listed below are specified in the Flags
    field beginning with the least significant bit, i.e the Encryption
    bit is bit 0 of the Flags field, the Commit bit is bit 1 of the Flags
    field, and the Authentication Only bit is bit 2 of the Flags field.
    The remaining bits of the Flags field MUST be set to 0 prior to
    transmission.


    --  E(ncryption Bit) (1 bit) - If set (1), all payloads following the
        header are encrypted using the encryption algorithm identified in
        the ISAKMP SA. The ISAKMP SA Identifier is the combination of the
        initiator and responder cookie.  It is RECOMMENDED that
        encryption of communications be done as soon as possible between
        the peers.  For all ISAKMP exchanges described in section 4.1,
        the encryption SHOULD begin after both parties have exchanged Key
        Exchange payloads.  If the E(ncryption Bit) is not set (0), the
        payloads are not encrypted.

    --  C(ommit Bit) (1 bit) - This bit is used to signal key exchange
        synchronization.  It is used to ensure that encrypted material is
        not received prior to completion of the SA establishment.  The


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        Commit Bit can be set (at anytime) by either party participating
        in the SA establishment, and can be used during both phases of an
        ISAKMP SA establishment.  However, the value MUST be reset after
        the Phase 1 negotiation.  If set(1), the entity which did not set
        the Commit Bit MUST wait for an Informational Exchange containing
        a Notify payload (with the CONNECTED Notify Message) from the en-
        tity which set the Commit Bit.  This indicates that the SA estab-
        lishment was successful and either entity can now proceed with en-
        crypted traffic communication.  In addition to synchronizing key ex-
        change, the Commit Bit can be used to protect against loss of trans-
        missions over unreliable networks and guard against the need for mul-
        tiple retransmissions.

        NOTE: It is always possible that the final message of an exchange
        can be lost.  In this case, the entity expecting to receive the
        final message of an exchange would receive the Phase 2 SA negoti-
        ation message following a Phase 1 exchange or encrypted traffic
        following a Phase 2 exchange.  Handling of this situation is not
        standardized, but we propose the following possibilities.  If the
        entity awaiting the Informational Exchange can verify the re-
        ceived message (i.e.  Phase 2 SA negotiation message or encrypted
        traffic), then they MAY consider the SA was established and
        continue processing.  The other option is to retransmit the last
        ISAKMP message to force the other entity to retransmit the final mes-
        sage.  This suggests that implementations may consider retaining the
        last message (locally) until they are sure the SA is established.

    --  A(uthentication Only Bit) (1 bit) - This bit is intended for use
        with the Informational Exchange with a Notify payload and will
        allow the transmission of information with integrity checking,
        but no encryption (e.g.  "emergency mode").  Section 4.8 states
        that a Phase 2 Informational Exchange MUST be sent under the
        protection of an ISAKMP SA. This is the only exception to that
        policy.  If the Authentication Only bit is set (1), only
        authentication security services will be applied to the entire
        Notify payload of the Informational Exchange and the payload will
        not be encrypted.



 o  Message ID (4 octets) - Unique Message Identifier used to identify
    protocol state during Phase 2 negotiations.  This value is randomly
    generated by the initiator of the Phase 2 negotiation.  In the event
    of simultaneous SA establishments (i.e.  collisions), the value of
    this field will likely be different because they are independently
    generated and, thus, two security associations will progress toward
    establishment.  However, it is unlikely there will be absolute
    simultaneous establishments.  During Phase 1 negotiations, the value
    MUST be set to 0.

 o  Length (4 octets) - Length of total message (header + payloads) in
    octets.  Encryption can expand the size of an ISAKMP message.

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3.2 Generic Payload Header


Each ISAKMP payload defined in sections 3.4 through 3.16 begins with a
generic header, shown in Figure 3, which provides a payload "chaining"
capability and clearly defines the boundaries of a payload.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 3:  Generic Payload Header

The Generic Payload Header fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.  This field provides the
    "chaining" capability.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.



3.3 Data Attributes


There are several instances within ISAKMP where it is necessary to repre-
sent Data Attributes.  An example of this is the Security Association (SA)
Attributes contained in the Transform payload (described in section 3.6).
These Data Attributes are not an ISAKMP payload, but are contained within
ISAKMP payloads.  The format of the Data Attributes provides the flexibil-
ity for representation of many different types of information.  There can
be multiple Data Attributes within a payload.  The length of the Data At-
tributes will either be 4 octets or defined by the Attribute Length field.
This is done using the Attribute Format bit described below.  Specific in-
formation about the attributes for each domain will be described in a DOI
document, e.g.  IPSEC DOI [IPDOI].

The Data Attributes fields are defined as follows:


 o  Attribute Type (2 octets) - Unique identifier for each type of
    attribute.  These attributes are defined as part of the DOI-specific

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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !A!       Attribute Type        !    AF=0  Attribute Length     !
        !F!                             !    AF=1  Attribute Value      !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        .                   AF=0  Attribute Value                       .
        .                   AF=1  Not Transmitted                       .
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                        Figure 4:  Data Attributes


    information.

    The most significant bit, or Attribute Format (AF), indicates whether
    the data attributes follow the Type/Length/Value (TLV) format or a
    shortened Type/Value (TV) format.  If the AF bit is a zero (0), then
    the Data Attributes are of the Type/Length/Value (TLV) form.  If the
    AF bit is a one (1), then the Data Attributes are of the Type/Value
    form.

 o  Attribute Length (2 octets) - Length in octets of the Attribute
    Value.  When the AF bit is a one (1), the Attribute Value is only 2
    octets and the Attribute Length field is not present.

 o  Attribute Value (variable length) - Value of the attribute associated
    with the DOI-specific Attribute Type.  If the AF bit is a zero (0),
    this field has a variable length defined by the Attribute Length
    field.  If the AF bit is a one (1), the Attribute Value has a length
    of 2 octets.


3.4 Security Association Payload


The Security Association Payload is used to negotiate security attributes
and to indicate the Domain of Interpretation (DOI) and Situation under
which the negotiation is taking place.  Figure 5 shows the format of the
Security Association payload.

The Security Association Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.  This field MUST NOT contain the
    values for the Proposal or Transform payloads as they are considered
    part of the security association negotiation.  For example, this


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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !              Domain of Interpretation  (DOI)                  !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                           Situation                           ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 5:  Security Association Payload


    field would contain the value "10" (Nonce payload) in the first
    message of a Base Exchange (see Section 4.4) and the value "0" in the
    first message of an Identity Protect Exchange (see Section 4.5).

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the entire Security
    Association payload, including the SA payload, all Proposal payloads,
    and all Transform payloads associated with the proposed Security
    Association.

 o  Domain of Interpretation (4 octets) - Identifies the DOI (as
    described in Section 2.1) under which this negotiation is taking
    place.  The DOI is a 32-bit unsigned integer.  A DOI value of 0
    during a Phase 1 exchange specifies a Generic ISAKMP SA which can be
    used for any protocol during the Phase 2 exchange.  The necessary SA
    Attributes are defined in A.4.  A DOI value of 1 is assigned to the
    IPsec DOI [IPDOI].  All other DOI values are reserved to IANA for
    future use.  IANA will not normally assign a DOI value without
    referencing some public specification, such as an Internet RFC. Other
    DOI's can be defined using the description in appendix B.  This field
    MUST be present within the Security Association payload.

 o  Situation (variable length) - A DOI-specific field that identifies
    the situation under which this negotiation is taking place.  The
    Situation is used to make policy decisions regarding the security
    attributes being negotiated.  Specifics for the IETF IP Security DOI
    Situation are detailed in [IPDOI].  This field MUST be present within
    the Security Association payload.


The payload type for the Security Association Payload is one (1).




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3.5 Proposal Payload


The Proposal Payload contains information used during Security Associa-
tion negotiation.  The proposal consists of security mechanisms, or trans-
forms, to be used to secure the communications channel.  Figure 6 shows
the format of the Proposal Payload.  A description of its use can be found
in section 4.2.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Proposal #   !  Protocol-Id  !    SPI Size   !# of Transforms!
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                        SPI (variable)                         !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 6:  Proposal Payload Format

The Proposal Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  This field MUST only contain the value "2"
    or "0".  If there are additional Proposal payloads in the message,
    then this field will be 2.  If the current Proposal payload is the
    last within the security association proposal, then this field will
    be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the entire Proposal
    payload, including generic payload header, the Proposal payload, and
    all Transform payloads associated with this proposal.  In the event
    there are multiple proposals with the same proposal number (see
    section 4.2), the Payload Length field only applies to the current
    Proposal payload and not to all Proposal payloads.

 o  Proposal # (1 octet) - Identifies the Proposal number for the current
    payload.  A description of the use of this field is found in section
    4.2.

 o  Protocol-Id (1 octet) - Specifies the protocol identifier for the
    current negotiation.  Examples might include IPSEC ESP, IPSEC AH,
    OSPF, TLS, etc.

 o  SPI Size (1 octet) - Length in octets of the SPI as defined by the


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    Protocol-Id.  In the case of ISAKMP, the Initiator and Responder
    cookie pair from the ISAKMP Header is the ISAKMP SPI, therefore, the
    SPI Size is irrelevant and MAY be from zero (0) to sixteen (16).  If
    the SPI Size is non-zero, the content of the SPI field MUST be
    ignored.  If the SPI Size is not a multiple of 4 octets it will have
    some impact on the SPI field and the alignment of all payloads in the
    message.  The Domain of Interpretation (DOI) will dictate the SPI
    Size for other protocols.

 o  # of Transforms (1 octet) - Specifies the number of transforms for
    the Proposal.  Each of these is contained in a Transform payload.

 o  SPI (variable) - The sending entity's SPI. In the event the SPI Size
    is not a multiple of 4 octets, there is no padding applied to the
    payload, however, it can be applied at the end of the message.


The payload type for the Proposal Payload is two (2).



3.6 Transform Payload


The Transform Payload contains information used during Security Associa-
tion negotiation.  The Transform payload consists of a specific security
mechanism, or transforms, to be used to secure the communications chan-
nel.  The Transform payload also contains the security association at-
tributes associated with the specific transform.  These SA attributes are
DOI-specific.  Figure 7 shows the format of the Transform Payload.  A de-
scription of its use can be found in section 4.2.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Transform #  !  Transform-Id !           RESERVED2           !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                        SA Attributes                          ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 7:  Transform Payload Format

The Transform Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next

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    payload in the message.  This field MUST only contain the value "3"
    or "0".  If there are additional Transform payloads in the proposal,
    then this field will be 3.  If the current Transform payload is the
    last within the proposal, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header, Transform values, and all SA
    Attributes.

 o  Transform # (1 octet) - Identifies the Transform number for the
    current payload.  If there is more than one transform proposed for a
    specific protocol within the Proposal payload, then each Transform
    payload has a unique Transform number.  A description of the use of
    this field is found in section 4.2.

 o  Transform-Id (1 octet) - Specifies the Transform identifier for the
    protocol within the current proposal.  These transforms are defined
    by the DOI and are dependent on the protocol being negotiated.

 o  RESERVED2 (2 octets) - Unused, set to 0.

 o  SA Attributes (variable length) - This field contains the security
    association attributes as defined for the transform given in the
    Transform-Id field.  The SA Attributes SHOULD be represented using
    the Data Attributes format described in section 3.3.  If the SA
    Attributes are not aligned on 4-byte boundaries, then subsequent
    payloads will not be aligned and any padding will be added at the end
    of the message to make the message 4-octet aligned.


The payload type for the Transform Payload is three (3).



3.7 Key Exchange Payload


The Key Exchange Payload supports a variety of key exchange techniques.
Example key exchanges are Oakley [Oakley], Diffie-Hellman, the enhanced
Diffie-Hellman key exchange described in X9.42 [ANSI], and the RSA-based
key exchange used by PGP. Figure 8 shows the format of the Key Exchange
payload.

The Key Exchange Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.


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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                       Key Exchange Data                       ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 8:  Key Exchange Payload Format


 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Key Exchange Data (variable length) - Data required to generate a
    session key.  The interpretation of this data is specified by the DOI
    and the associated Key Exchange algorithm.  This field may also
    contain pre-placed key indicators.


The payload type for the Key Exchange Payload is four (4).


3.8 Identification Payload


The Identification Payload contains DOI-specific data used to exchange
identification information.  This information is used for determining the
identities of communicating peers and may be used for determining authen-
ticity of information.  Figure 9 shows the format of the Identification
Payload.

The Identification Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  ID Type (1 octet) - Specifies the type of Identification being used.


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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !   ID Type     !             DOI Specific ID Data              !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                   Identification Data                         ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 9:  Identification Payload Format


    This field is DOI-dependent.

 o  DOI Specific ID Data (3 octets) - Contains DOI specific
    Identification data.  If unused, then this field MUST be set to 0.

 o  Identification Data (variable length) - Contains identity
    information.  The values for this field are DOI-specific and the
    format is specified by the ID Type field.  Specific details for the
    IETF IP Security DOI Identification Data are detailed in [IPDOI].


The payload type for the Identification Payload is five (5).


3.9 Certificate Payload


The Certificate Payload provides a means to transport certificates or
other certificate-related information via ISAKMP and can appear in any
ISAKMP message.  Certificate payloads SHOULD be included in an exchange
whenever an appropriate directory service (e.g.  Secure DNS [DNSSEC]) is
not available to distribute certificates.  The Certificate payload MUST be
accepted at any point during an exchange.  Figure 10 shows the format of
the Certificate Payload.

NOTE: Certificate types and formats are not generally bound to a DOI - it
is expected that there will only be a few certificate types, and that most
DOIs will accept all of these types.

The Certificate Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the


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                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Cert Encoding !                                               !
        +-+-+-+-+-+-+-+-+                                               !
        ~                       Certificate Data                        ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 10:  Certificate Payload Format


    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Certificate Encoding (1 octet) - This field indicates the type of
    certificate or certificate-related information contained in the
    Certificate Data field.


                _________Certificate_Type____________Value____
                 NONE                                  0
                 PKCS #7 wrapped X.509 certificate      1
                 PGP Certificate                        2
                 DNS Signed Key                         3
                 X.509 Certificate - Signature          4
                 X.509 Certificate - Key Exchange       5
                 Kerberos Tokens                        6
                 Certificate Revocation List (CRL)      7
                 Authority Revocation List (ARL)        8
                 SPKI Certificate                       9
                 X.509 Certificate - Attribute         10
                 RESERVED                           11 - 255



 o  Certificate Data (variable length) - Actual encoding of certificate
    data.  The type of certificate is indicated by the Certificate
    Encoding field.


The payload type for the Certificate Payload is six (6).



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3.10 Certificate Request Payload


The Certificate Request Payload provides a means to request certificates
via ISAKMP and can appear in any message.  Certificate Request payloads
SHOULD be included in an exchange whenever an appropriate directory ser-
vice (e.g.  Secure DNS [DNSSEC]) is not available to distribute certifi-
cates.  The Certificate Request payload MUST be accepted at any point dur-
ing the exchange.  The responder to the Certificate Request payload MUST
send its certificate, if certificates are supported, based on the values
contained in the payload.  If multiple certificates are required, then
multiple Certificate Request payloads SHOULD be transmitted.  Figure 11
shows the format of the Certificate Request Payload.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Cert. Type   !                                               !
        +-+-+-+-+-+-+-+-+                                               !
        ~                    Certificate Authority                      ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



              Figure 11:  Certificate Request Payload Format

The Certificate Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Certificate Type (1 octet) - Contains an encoding of the type of
    certificate requested.  Acceptable values are listed in section 3.9.

 o  Certificate Authority (variable length) - Contains an encoding of an
    acceptable certificate authority for the type of certificate
    requested.  As an example, for an X.509 certificate this field would
    contain the Distinguished Name encoding of the Issuer Name of an
    X.509 certificate authority acceptable to the sender of this payload.
    This would be included to assist the responder in determining how


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    much of the certificate chain would need to be sent in response to
    this request.  If there is no specific certificate authority
    requested, this field SHOULD not be included.


The payload type for the Certificate Request Payload is seven (7).



3.11 Hash Payload


The Hash Payload contains data generated by the hash function (selected
during the SA establishment exchange), over some part of the message
and/or ISAKMP state.  This payload may be used to verify the integrity of
the data in an ISAKMP message or for authentication of the negotiating en-
tities.  Figure 12 shows the format of the Hash Payload.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                           Hash Data                           ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 12:  Hash Payload Format

The Hash Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Hash Data (variable length) - Data that results from applying the
    hash routine to the ISAKMP message and/or state.


The payload type for the Hash Payload is eight (8).




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3.12 Signature Payload


The Signature Payload contains data generated by the digital signature
function (selected during the SA establishment exchange), over some part
of the message and/or ISAKMP state.  This payload is used to verify the
integrity of the data in the ISAKMP message, and may be of use for non-
repudiation services.  Figure 13 shows the format of the Signature Pay-
load.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                         Signature Data                        ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 13:  Signature Payload Format

The Signature Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Signature Data (variable length) - Data that results from applying
    the digital signature function to the ISAKMP message and/or state.


The payload type for the Signature Payload is nine (9).



3.13 Nonce Payload


The Nonce Payload contains random data used to guarantee liveness dur-
ing an exchange and protect against replay attacks.  Figure 14 shows the
format of the Nonce Payload.  If nonces are used by a particular key ex-
change, the use of the Nonce payload will be dictated by the key exchange.
The nonces may be transmitted as part of the key exchange data, or as a

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separate payload.  However, this is defined by the key exchange, not by
ISAKMP.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                            Nonce Data                         ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 14:  Nonce Payload Format

The Nonce Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Nonce Data (variable length) - Contains the random data generated by
    the transmitting entity.


The payload type for the Nonce Payload is ten (10).



3.14 Notification Payload


The Notification Payload can contain both ISAKMP and DOI-specific data and
is used to transmit informational data, such as error conditions, to an
ISAKMP peer.  It is possible to send multiple Notification payloads in
a single ISAKMP message.  Figure 15 shows the format of the Notification
Payload.

Notification which occurs during, or is concerned with, a Phase 1 nego-
tiation is identified by the Initiator and Responder cookie pair in the
ISAKMP Header.  The Protocol Identifier, in this case, is ISAKMP and the
SPI value is 0 because the cookie pair in the ISAKMP Header identifies the
ISAKMP SA. If the notification takes place prior to the completed exchange
of keying information, then the notification will be unprotected.

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Notification which occurs during, or is concerned with, a Phase 2 nego-
tiation is identified by the Initiator and Responder cookie pair in the
ISAKMP Header and the Message ID and SPI associated with the current nego-
tiation.  One example for this type of notification is to indicate why a
proposal was rejected.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !              Domain of Interpretation  (DOI)                  !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Protocol-ID  !   SPI Size    !      Notify Message Type      !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                Security Parameter Index (SPI)                 ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                       Notification Data                       ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 15:  Notification Payload Format

The Notification Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Domain of Interpretation (4 octets) - Identifies the DOI (as
    described in Section 2.1) under which this notification is taking
    place.  For ISAKMP this value is zero (0) and for the IPSEC DOI it is
    one (1).  Other DOI's can be defined using the description in
    appendix B.

 o  Protocol-Id (1 octet) - Specifies the protocol identifier for the
    current notification.  Examples might include ISAKMP, IPSEC ESP,
    IPSEC AH, OSPF, TLS, etc.

 o  SPI Size (1 octet) - Length in octets of the SPI as defined by the


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    Protocol-Id.  In the case of ISAKMP, the Initiator and Responder
    cookie pair from the ISAKMP Header is the ISAKMP SPI, therefore, the
    SPI Size is irrelevant and MAY be from zero (0) to sixteen (16).  If
    the SPI Size is non-zero, the content of the SPI field MUST be
    ignored.  The Domain of Interpretation (DOI) will dictate the SPI
    Size for other protocols.

 o  Notify Message Type (2 octets) - Specifies the type of notification
    message (see section 3.14.1).  Additional text, if specified by the
    DOI, is placed in the Notification Data field.

 o  SPI (variable length) - Security Parameter Index.  The receiving
    entity's SPI. The use of the SPI field is described in section 2.4.
    The length of this field is determined by the SPI Size field and is
    not necessarily aligned to a 4 octet boundary.

 o  Notification Data (variable length) - Informational or error data
    transmitted in addition to the Notify Message Type.  Values for this
    field are DOI-specific.


The payload type for the Notification Payload is eleven (11).


3.14.1 Notify Message Types


Notification information can be error messages specifying why an SA could
not be established.  It can also be status data that a process managing
an SA database wishes to communicate with a peer process.  For example,
a secure front end or security gateway may use the Notify message to syn-
chronize SA communication.  The table below lists the Nofitication mes-
sages and their corresponding values.  Values in the Private Use range are
expected to be DOI-specific values.



















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                      NOTIFY MESSAGES - ERROR TYPES

                 __________Errors______________Value_____
                  INVALID-PAYLOAD-TYPE           1
                  DOI-NOT-SUPPORTED              2
                  SITUATION-NOT-SUPPORTED        3
                  INVALID-COOKIE                 4
                  INVALID-MAJOR-VERSION          5
                  INVALID-MINOR-VERSION          6
                  INVALID-EXCHANGE-TYPE          7
                  INVALID-FLAGS                  8
                  INVALID-MESSAGE-ID             9
                  INVALID-PROTOCOL-ID            10
                  INVALID-SPI                    11
                  INVALID-TRANSFORM-ID           12
                  ATTRIBUTES-NOT-SUPPORTED       13
                  NO-PROPOSAL-CHOSEN             14
                  BAD-PROPOSAL-SYNTAX            15
                  PAYLOAD-MALFORMED              16
                  INVALID-KEY-INFORMATION        17
                  INVALID-ID-INFORMATION         18
                  INVALID-CERT-ENCODING          19
                  INVALID-CERTIFICATE            20
                  CERT-TYPE-UNSUPPORTED          21
                  INVALID-CERT-AUTHORITY         22
                  INVALID-HASH-INFORMATION       23
                  AUTHENTICATION-FAILED          24
                  INVALID-SIGNATURE              25
                  ADDRESS-NOTIFICATION           26
                  NOTIFY-SA-LIFETIME             27
                  CERTIFICATE-UNAVAILABLE        28
                  RESERVED (Future Use)      29 - 8191
                  Private Use              8192 - 16383



                      NOTIFY MESSAGES - STATUS TYPES
                 _________Status_____________Value______
                  CONNECTED                   16384
                  RESERVED (Future Use)   16385 - 24575
                  DOI-specific codes     24576 - 32767
                  Private Use            32768 - 40959
                  RESERVED (Future Use)  40960 - 65535


3.15 Delete Payload


The Delete Payload contains a protocol-specific security association iden-
tifier that the sender has removed from its security association database
and is, therefore, no longer valid.  Figure 16 shows the format of the
Delete Payload.  It is possible to send multiple SPIs in a Delete payload,

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however, each SPI MUST be for the same protocol.  Mixing of Protocol Iden-
tifiers MUST NOT be performed with the Delete payload.

Deletion which is concerned with an ISAKMP SA will contain a Protocol-Id
of ISAKMP and the SPIs are the initiator and responder cookies from the
ISAKMP Header.  Deletion which is concerned with a Protocol SA, such as
ESP or AH, will contain the Protocol-Id of that protocol (e.g.  ESP, AH)
and the SPI is the sending entity's SPI(s).

NOTE: The Delete Payload is not a request for the responder to delete an
SA, but an advisory from the initiator to the responder.  If the responder
chooses to ignore the message, the next communication from the responder
to the initiator, using that security association, will fail.  A responder
is not expected to acknowledge receipt of a Delete payload.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !              Domain of Interpretation  (DOI)                  !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !  Protocol-Id  !   SPI Size    !           # of SPIs           !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~               Security Parameter Index(es) (SPI)              ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 16:  Delete Payload Format

The Delete Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Domain of Interpretation (4 octets) - Identifies the DOI (as
    described in Section 2.1) under which this deletion is taking place.
    For ISAKMP this value is zero (0) and for the IPSEC DOI it is one
    (1).  Other DOI's can be defined using the description in appendix B.

 o  Protocol-Id (1 octet) - ISAKMP can establish security associations


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    for various protocols, including ISAKMP and IPSEC. This field identi-
    fies which security association database to apply the delete request.

 o  SPI Size (1 octet) - Length in octets of the SPI as defined by the
    Protocol-Id.  In the case of ISAKMP, the Initiator and Responder
    cookie pair is the ISAKMP SPI. In this case, the SPI Size would be 16
    octets for each SPI being deleted.

 o  # of SPIs (2 octets) - The number of SPIs contained in the Delete
    payload.  The size of each SPI is defined by the SPI Size field.

 o  Security Parameter Index(es) (variable length) - Identifies the
    specific security association(s) to delete.  Values for this field
    are DOI and protocol specific.  The length of this field is
    determined by the SPI Size and # of SPIs fields.


The payload type for the Delete Payload is twelve (12).



3.16 Vendor ID Payload


The Vendor ID Payload contains a vendor defined constant.  The constant
is used by vendors to identify and recognize remote instances of their
implementations.  This mechanism allows a vendor to experiment with new
features while maintaining backwards compatibility.  This is not a general
extension facility of ISAKMP. Figure 17 shows the format of the Vendor ID
Payload.

The Vendor ID payload is not an announcement from the sender that it will
send private payload types.  A vendor sending the Vendor ID MUST not make
any assumptions about private payloads that it may send unless a Vendor ID
is received as well.  Multiple Vendor ID payloads MAY be sent.  An imple-
mentation is NOT REQUIRED to understand any Vendor ID payloads.  An imple-
mentation is NOT REQUIRED to send any Vendor ID payload at all.  If a pri-
vate payload was sent without prior agreement to send it, a compliant im-
plementation may reject a proposal with a notify message of type INVALID-
PAYLOAD-TYPE.

If a Vendor ID payload is sent, it MUST be sent during the Phase 1 negoti-
ation.  Reception of a familiar Vendor ID payload in the Phase 1 negotia-
tion allows an implementation to make use of Private USE payload numbers
(128-255), described in section 3.1 for vendor specific extensions during
Phase 2 negotiations.  The definition of "familiar" is left to implementa-
tions to determine.  Some vendors may wish to implement another vendor's
extension prior to standardization.  However, this practice SHOULD not be
widespread and vendors should work towards standardization instead.

The vendor defined constant MUST be unique.  The choice of hash and text
to hash is left to the vendor to decide.  As an example, vendors could

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generate their vendor id by taking a plain (non-keyed) hash of a string
containing the product name, and the version of the product.  A hash is
used instead of a vendor registry to avoid local cryptographic policy
problems with having a list of "approved" products, to keep away from
maintaining a list of vendors, and to allow classified products to avoid
having to appear on any list.  For instance:

"Example Company IPsec.  Version 97.1"

(not including the quotes) has MD5 hash:
48544f9b1fe662af98b9b39e50c01a5a, when using MD5file.  Vendors may include
all of the hash, or just a portion of it, as the payload length will bound
the data.  There are no security implications of this hash, so its choice
is arbitrary.


                             1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ! Next Payload  !   RESERVED    !         Payload Length        !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        !                                                               !
        ~                        Vendor ID (VID)                        ~
        !                                                               !
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 17:  Vendor ID Payload Format

The Vendor ID Payload fields are defined as follows:


 o  Next Payload (1 octet) - Identifier for the payload type of the next
    payload in the message.  If the current payload is the last in the
    message, then this field will be 0.

 o  RESERVED (1 octet) - Unused, set to 0.

 o  Payload Length (2 octets) - Length in octets of the current payload,
    including the generic payload header.

 o  Vendor ID (variable length) - Hash of the vendor string plus version
    (as described above).


The payload type for the Vendor ID Payload is thirteen (13).







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4 ISAKMP Exchanges


ISAKMP supplies the basic syntax of a message exchange.  The basic build-
ing blocks for ISAKMP messages are the payload types described in section
3.  This section describes the procedures for SA establishment and SA mod-
ification, followed by a default set of exchanges that MAY be used for
initial interoperability.  Other exchanges will be defined depending on
the DOI and key exchange.  [IPDOI] and [IKE] are examples of how this is
achieved.  Appendix B explains the procedures for accomplishing these ad-
ditions.



4.1 ISAKMP Exchange Types


ISAKMP allows the creation of exchanges for the establishment of Security
Associations and keying material.  There are currently five default Ex-
change Types defined for ISAKMP. Sections 4.4 through 4.8 describe these
exchanges.  Exchanges define the content and ordering of ISAKMP messages
during communications between peers.  Most exchanges will include all the
basic payload types - SA, KE, ID, SIG - and may include others.  The pri-
mary difference between exchange types is the ordering of the messages and
the payload ordering within each message.  While the ordering of payloads
within messages is not mandated, for processing efficiency it is RECOM-
MENDED that the Security Association payload be the first payload within
an exchange.  Processing of each payload within an exchange is described
in section 5.

Sections 4.4 through 4.8 provide a default set of ISAKMP exchanges.  These
exchanges provide different security protection for the exchange itself
and information exchanged.  The diagrams in each of the following sections
show the message ordering for each exchange type as well as the payloads
included in each message, and provide basic notes describing what has hap-
pened after each message exchange.  None of the examples include any "op-
tional payloads", like certificate and certificate request.  Additionally,
none of the examples include an initial exchange of ISAKMP Headers (con-
taining initiator and responder cookies) which would provide protection
against clogging (see section 2.5.3).

The defined exchanges are not meant to satisfy all DOI and key exchange
protocol requirements.  If the defined exchanges meet the DOI require-
ments, then they can be used as outlined.  If the defined exchanges do
not meet the security requirements defined by the DOI, then the DOI MUST
specify new exchange type(s) and the valid sequences of payloads that make
up a successful exchange, and how to build and interpret those payloads.
All ISAKMP implementations MUST implement the Informational Exchange and
SHOULD implement the other four exchanges.  However, this is dependent on
the definition of the DOI and associated key exchange protocols.

As discussed above, these exchange types can be used in either phase of

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negotiation.  However, they may provide different security properties
in each of the phases.  With each of these exchanges, the combination of
cookies and SPI fields identifies whether this exchange is being used in
the first or second phase of a negotiation.


4.1.1 Notation


The following notation is used to describe the ISAKMP exchange types,
shown in the next section, with the message formats and associated pay-
loads:




HDR is an ISAKMP header whose exchange type defines the payload orderings
     SA is an SA negotiation payload with one or more Proposal and
        Transform payloads. An initiator MAY provide multiple proposals
          for negotiation; a responder MUST reply with only one.
     KE is the key exchange payload.
     IDx is the identity payload for "x". x can be: "ii" or "ir"
          for the ISAKMP initiator and responder, respectively, or x can
          be: "ui", "ur" (when the ISAKMP daemon is a proxy negotiator),
          for the user initiator and responder, respectively.
     HASH is the hash payload.
     SIG is the signature payload. The data to sign is exchange-specific.
     AUTH is a generic authentication mechanism, such as HASH or SIG.
     NONCE is the nonce payload.
     '*' signifies payload encryption after the ISAKMP header. This
          encryption MUST begin immediately after the ISAKMP header and
          all payloads following the ISAKMP header MUST be encrypted.

     => signifies "initiator to responder" communication
     <= signifies "responder to initiator" communication


4.2 Security Association Establishment


The Security Association, Proposal, and Transform payloads are used to
build ISAKMP messages for the negotiation and establishment of SAs.  An
SA establishment message consists of a single SA payload followed by at
least one, and possibly many, Proposal payloads and at least one, and pos-
sibly many, Transform payloads associated with each Proposal payload.  Be-
cause these payloads are considered together, the SA payload will point to
any following payloads and not to the Proposal payload included with the
SA payload.  The SA Payload contains the DOI and Situation for the pro-
posed SA. Each Proposal payload contains a Security Parameter Index (SPI)
and ensures that the SPI is associated with the Protocol-Id in accordance
with the Internet Security Architecture [RFC-1825].  Proposal payloads may
or may not have the same SPI, as this is implementation dependent.  Each

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Transform Payload contains the specific security mechanisms to be used for
the designated protocol.  It is expected that the Proposal and Transform
payloads will be used only during SA establishment negotiation.  The cre-
ation of payloads for security association negotiation and establishment
described here in this section are applicable for all ISAKMP exchanges de-
scribed later in sections 4.4 through 4.8.  The examples shown in 4.2.1
contain only the SA, Proposal, and Transform payloads and do not contain
other payloads that might exist for a given ISAKMP exchange.

The Proposal payload provides the initiating entity with the capability
to present to the responding entity the security protocols and associated
security mechanisms for use with the security association being negoti-
ated.  If the SA establishment negotiation is for a combined protection
suite consisting of multiple protocols, then there MUST be multiple Pro-
posal payloads each with the same Proposal number.  These proposals MUST
be considered as a unit and MUST NOT be separated by a proposal with a
different proposal number.  The use of the same Proposal number in mul-
tiple Proposal payloads provides a logical AND operation, i.e.  Protocol
1 AND Protocol 2.  The first example below shows an ESP AND AH protection
suite.  If the SA establishment negotiation is for different protection
suites, then there MUST be multiple Proposal payloads each with a monoton-
ically increasing Proposal number.  The different proposals MUST be pre-
sented in the initiator's preference order.  The use of different Proposal
numbers in multiple Proposal payloads provides a logical OR operation,
i.e.  Proposal 1 OR Proposal 2, where each proposal may have more than one
protocol.  The second example below shows either an AH AND ESP protection
suite OR just an ESP protection suite.  Note that the Next Payload field
of the Proposal payload points to another Proposal payload (if it exists).
The existence of a Proposal payload implies the existence of one or more
Transform payloads.

The Transform payload provides the initiating entity with the capability
to present to the responding entity multiple mechanisms, or transforms,
for a given protocol.  The Proposal payload identifies a Protocol for
which services and mechanisms are being negotiated.  The Transform pay-
load allows the initiating entity to present several possible supported
transforms for that proposed protocol.  There may be several transforms
associated with a specific Proposal payload each identified in a separate
Transform payload.  The multiple transforms MUST be presented with mono-
tonically increasing numbers in the initiator's preference order.  The
receiving entity MUST select a single transform for each protocol in a
proposal or reject the entire proposal.  The use of the Transform num-
ber in multiple Transform payloads provides a second level OR operation,
i.e.  Transform 1 OR Transform 2 OR Transform 3.  Example 1 below shows
two possible transforms for ESP and a single transform for AH. Example 2
below shows one transform for AH AND one transform for ESP OR two trans-
forms for ESP alone.  Note that the Next Payload field of the Transform
payload points to another Transform payload or 0.  The Proposal payload
delineates the different proposals.

When responding to a Security Association payload, the responder MUST send


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a Security Association payload with the selected proposal, which may con-
sist of multiple Proposal payloads and their associated Transform pay-
loads.  Each of the Proposal payloads MUST contain a single Transform
payload associated with the Protocol.  The responder SHOULD retain the
Proposal # field in the Proposal payload and the Transform # field in
each Transform payload of the selected Proposal.  Retention of Proposal
and Transform numbers should speed the initiator's protocol processing by
negating the need to compare the respondor's selection with every offered
option.  These values enable the initiator to perform the comparison di-
rectly and quickly.  The initiator MUST verify that the Security Associa-
tion payload received from the responder matches one of the proposals sent
initially.


4.2.1 Security Association Establishment Examples


This example shows a Proposal for a combined protection suite with two
different protocols.  The first protocol is presented with two transforms
supported by the proposer.  The second protocol is presented with a sin-
gle transform.  An example for this proposal might be:  Protocol 1 is ESP
with Transform 1 as 3DES and Transform 2 as DES AND Protocol 2 is AH with
Transform 1 as SHA. The responder MUST select from the two transforms pro-
posed for ESP. The resulting protection suite will be either (1) 3DES AND
SHA OR (2) DES AND SHA, depending on which ESP transform was selected by
the responder.  Note this example is shown using the Base Exchange.



                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      /+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Nonce    !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SA Pay !                 Domain of Interpretation (DOI)                !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                           Situation                           !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Proposal !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prop 1 ! Proposal # = 1!  Protocol-Id  !    SPI Size   !# of Trans. = 2!
Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SPI (variable)                        !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Transform!   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 1 ! Transform # 1 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Tran 2 ! Transform # 2 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prop 1 ! Proposal # = 1!  Protocol ID  !    SPI Size   !# of Trans. = 1!
Prot 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SPI (variable)                        !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 1 ! Transform # 1 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      \+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




This second example shows a Proposal for two different protection suites.
The SA Payload was omitted for space reasons.  The first protection suite
is presented with one transform for the first protocol and one transform
for the second protocol.  The second protection suite is presented with
two transforms for a single protocol.  An example for this proposal might
be:  Proposal 1 with Protocol 1 as AH with Transform 1 as MD5 AND Protocol
2 as ESP with Transform 1 as 3DES. This is followed by Proposal 2 with
Protocol 1 as ESP with Transform 1 as DES and Transform 2 as 3DES. The
responder MUST select from the two different proposals.  If the second
Proposal is selected, the responder MUST select from the two transforms
for ESP. The resulting protection suite will be either (1) MD5 AND 3DES OR
the selection between (2) DES OR (3) 3DES.


                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      /+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Proposal !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prop 1 ! Proposal # = 1!  Protocol ID  !    SPI Size   !# of Trans. = 1!
Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SPI (variable)                        !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 1 ! Transform # 1 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Proposal !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prop 1 ! Proposal # = 1! Protocol ID   !    SPI Size   !# of Trans. = 1!

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Prot 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SPI (variable)                        !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 1 ! Transform # 1 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prop 2 ! Proposal # = 2! Protocol ID   !    SPI Size   !# of Trans. = 2!
Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SPI (variable)                        !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = Transform!   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 1 ! Transform # 1 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / ! NP = 0        !   RESERVED    !         Payload Length        !
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tran 2 ! Transform # 2 ! Transform ID  !           RESERVED2           !
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ !                         SA Attributes                         !
      \+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




4.3 Security Association Modification


Security Association modification within ISAKMP is accomplished by cre-
ating a new SA and initiating communications using that new SA. Deletion
of the old SA can be done anytime after the new SA is established.  Dele-
tion of the old SA is dependent on local security policy.  Modification of
SAs by using a "Create New SA followed by Delete Old SA" method is done to
avoid potential vulnerabilities in synchronizing modification of existing
SA attributes.  The procedure for creating new SAs is outlined in section
4.2.  The procedure for deleting SAs is outlined in section 5.15.

Modification of an ISAKMP SA (phase 1 negotiation) follows the same proce-
dure as creation of an ISAKMP SA. There is no relationship between the two
SAs and the initiator and responder cookie pairs SHOULD be different, as
outlined in section 2.5.3.

Modification of a Protocol SA (phase 2 negotiation) follows the same pro-
cedure as creation of a Protocol SA. The creation of a new SA is protected
by the existing ISAKMP SA. There is no relationship between the two Proto-
col SAs.  A protocol implementation SHOULD begin using the newly created

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SA for outbound traffic and SHOULD continue to support incoming traffic
on the old SA until it is deleted or until traffic is received under the
protection of the newly created SA. As stated previously in this section,
deletion of an old SA is then dependent on local security policy.



4.4 Base Exchange


The Base Exchange is designed to allow the Key Exchange and Authentica-
tion related information to be transmitted together.  Combining the Key
Exchange and Authentication-related information into one message reduces
the number of round-trips at the expense of not providing identity pro-
tection.  Identity protection is not provided because identities are ex-
changed before a common shared secret has been established and, therefore,
encryption of the identities is not possible.  The following diagram shows
the messages with the possible payloads sent in each message and notes for
an example of the Base Exchange.



                                      BASE EXCHANGE

_#______Initiator____Direction_____Responder______________________NOTE____________________
(1)  HDR; SA; NONCE      =>                     Begin ISAKMP-SA or Proxy negotiation

(2)                      <=     HDR; SA; NONCE
                                                Basic SA agreed upon
(3)  HDR; KE;            =>
     IDii; AUTH                                 Key Generated (by responder)
                                                Initiator Identity Verified by Responder
(4)                      <=     HDR; KE;
                                IDir; AUTH
                                                Responder Identity Verified by Initiator
                                                Key Generated (by initiator)
                                                SA established


In the first message (1), the initiator generates a proposal it considers
adequate to protect traffic for the given situation.  The Security Associ-
ation, Proposal, and Transform payloads are included in the Security Asso-
ciation payload (for notation purposes).  Random information which is used
to guarantee liveness and protect against replay attacks is also trans-
mitted.  Random information provided by both parties SHOULD be used by the
authentication mechanism to provide shared proof of participation in the
exchange.

In the second message (2), the responder indicates the protection suite it
has accepted with the Security Association, Proposal, and Transform pay-
loads.  Again, random information which is used to guarantee liveness and
protect against replay attacks is also transmitted.  Random information

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provided by both parties SHOULD be used by the authentication mechanism
to provide shared proof of participation in the exchange.  Local secu-
rity policy dictates the action of the responder if no proposed protection
suite is accepted.  One possible action is the transmission of a Notify
payload as part of an Informational Exchange.

In the third (3) and fourth (4) messages, the initiator and responder, re-
spectively, exchange keying material used to arrive at a common shared
secret and identification information.  This information is transmitted
under the protection of the agreed upon authentication function.  Local
security policy dictates the action if an error occurs during these mes-
sages.  One possible action is the transmission of a Notify payload as
part of an Informational Exchange.



4.5 Identity Protection Exchange


The Identity Protection Exchange is designed to separate the Key Exchange
information from the Identity and Authentication related information.
Separating the Key Exchange from the Identity and Authentication related
information provides protection of the communicating identities at the ex-
pense of two additional messages.  Identities are exchanged under the pro-
tection of a previously established common shared secret.  The following
diagram shows the messages with the possible payloads sent in each message
and notes for an example of the Identity Protection Exchange.



                                 IDENTITY PROTECTION EXCHANGE

_#_______Initiator_____Direction______Responder_____NOTE________________________________________
(1)  HDR; SA               =>                       Begin ISAKMP-SA or Proxy negotiation
(2)                        <=     HDR; SA
                                                    Basic SA agreed upon
(3)  HDR; KE; NONCE        =>
(4)                        <=     HDR; KE; NONCE
                                                    Key Generated (by Initiator and Responder)
(5)  HDR*; IDii; AUTH      =>
                                                    Initiator Identity Verified by Responder
(6)                        <=     HDR*; IDir; AUTH
                                                    Responder Identity Verified by Initiator
                                                    SA established


In the first message (1), the initiator generates a proposal it consid-
ers adequate to protect traffic for the given situation.  The Security As-
sociation, Proposal, and Transform payloads are included in the Security
Association payload (for notation purposes).



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In the second message (2), the responder indicates the protection suite it
has accepted with the Security Association, Proposal, and Transform pay-
loads.  Local security policy dictates the action of the responder if no
proposed protection suite is accepted.  One possible action is the trans-
mission of a Notify payload as part of an Informational Exchange.

In the third (3) and fourth (4) messages, the initiator and responder, re-
spectively, exchange keying material used to arrive at a common shared se-
cret and random information which is used to guarantee liveness and pro-
tect against replay attacks.  Random information provided by both parties
SHOULD be used by the authentication mechanism to provide shared proof
of participation in the exchange.  Local security policy dictates the ac-
tion if an error occurs during these messages.  One possible action is the
transmission of a Notify payload as part of an Informational Exchange.

In the fifth (5) and sixth (6) messages, the initiator and responder, re-
spectively, exchange identification information and the results of the
agreed upon authentication function.  This information is transmitted un-
der the protection of the common shared secret.  Local security policy
dictates the action if an error occurs during these messages.  One pos-
sible action is the transmission of a Notify payload as part of an Infor-
mational Exchange.



4.6 Authentication Only Exchange


The Authentication Only Exchange is designed to allow only Authentication
related information to be transmitted.  The benefit of this exchange is
the ability to perform only authentication without the computational ex-
pense of computing keys.  Using this exchange during negotiation, none of
the transmitted information will be encrypted.  However, the information
may be encrypted in other places.  For example, if encryption is negoti-
ated during the first phase of a negotiation and the authentication only
exchange is used in the second phase of a negotiation, then the authenti-
cation only exchange will be encrypted by the ISAKMP SAs negotiated in the
first phase.  The following diagram shows the messages with possible pay-
loads sent in each message and notes for an example of the Authentication
Only Exchange.













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                               AUTHENTICATION ONLY EXCHANGE

_#______Initiator_____Direction_____Responder_______________________NOTE____________________
(1)  HDR; SA; NONCE       =>                      Begin ISAKMP-SA or Proxy negotiation

(2)                       <=     HDR; SA; NONCE;
                                 IDir; AUTH
                                                  Basic SA agreed upon
                                                  Responder Identity Verified by Initiator
(3)  HDR; IDii; AUTH      =>
                                                  Initiator Identity Verified by Responder
                                                  SA established


In the first message (1), the initiator generates a proposal it considers
adequate to protect traffic for the given situation.  The Security Associ-
ation, Proposal, and Transform payloads are included in the Security Asso-
ciation payload (for notation purposes).  Random information which is used
to guarantee liveness and protect against replay attacks is also trans-
mitted.  Random information provided by both parties SHOULD be used by the
authentication mechanism to provide shared proof of participation in the
exchange.

In the second message (2), the responder indicates the protection suite it
has accepted with the Security Association, Proposal, and Transform pay-
loads.  Again, random information which is used to guarantee liveness and
protect against replay attacks is also transmitted.  Random information
provided by both parties SHOULD be used by the authentication mechanism
to provide shared proof of participation in the exchange.  Additionally,
the responder transmits identification information.  All of this infor-
mation is transmitted under the protection of the agreed upon authentica-
tion function.  Local security policy dictates the action of the responder
if no proposed protection suite is accepted.  One possible action is the
transmission of a Notify payload as part of an Informational Exchange.

In the third message (3), the initiator transmits identification informa-
tion.  This information is transmitted under the protection of the agreed
upon authentication function.  Local security policy dictates the action
if an error occurs during these messages.  One possible action is the
transmission of a Notify payload as part of an Informational Exchange.



4.7 Aggressive Exchange


The Aggressive Exchange is designed to allow the Security Association, Key
Exchange and Authentication related payloads to be transmitted together.
Combining the Security Association, Key Exchange, and Authentication-
related information into one message reduces the number of round-trips at
the expense of not providing identity protection.  Identity protection is

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not provided because identities are exchanged before a common shared se-
cret has been established and, therefore, encryption of the identities is
not possible.  Additionally, the Aggressive Exchange is attempting to es-
tablish all security relevant information in a single exchange.  The fol-
lowing diagram shows the messages with possible payloads sent in each mes-
sage and notes for an example of the Aggressive Exchange.



                                   AGGRESSIVE EXCHANGE

_#_____Initiator___Direction______Responder________________________NOTE____________________
(1)  HDR; SA; KE;      =>                        Begin ISAKMP-SA or Proxy negotiation
     NONCE; IDii                                 and Key Exchange

(2)                    <=     HDR; SA; KE;
                              NONCE; IDir; AUTH
                                                 Initiator Identity Verified by Responder
                                                 Key Generated
                                                 Basic SA agreed upon
(3)  HDR*; AUTH        =>
                                                 Responder Identity Verified by Initiator
                                                 SA established



In the first message (1), the initiator generates a proposal it considers
adequate to protect traffic for the given situation.  The Security Associ-
ation, Proposal, and Transform payloads are included in the Security Asso-
ciation payload (for notation purposes).  There can be only one Proposal
and one Transform offered (i.e.  no choices) in order for the aggressive
exchange to work.  Keying material used to arrive at a common shared se-
cret and random information which is used to guarantee liveness and pro-
tect against replay attacks are also transmitted.  Random information pro-
vided by both parties SHOULD be used by the authentication mechanism to
provide shared proof of participation in the exchange.  Additionally, the
initiator transmits identification information.

In the second message (2), the responder indicates the protection suite
it has accepted with the Security Association, Proposal, and Transform
payloads.  Keying material used to arrive at a common shared secret and
random information which is used to guarantee liveness and protect against
replay attacks is also transmitted.  Random information provided by both
parties SHOULD be used by the authentication mechanism to provide shared
proof of participation in the exchange.  Additionally, the responder
transmits identification information.  All of this information is trans-
mitted under the protection of the agreed upon authentication function.
Local security policy dictates the action of the responder if no proposed
protection suite is accepted.  One possible action is the transmission of
a Notify payload as part of an Informational Exchange.



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In the third (3) message, the initiator transmits the results of the
agreed upon authentication function.  This information is transmitted un-
der the protection of the common shared secret.  Local security policy
dictates the action if an error occurs during these messages.  One pos-
sible action is the transmission of a Notify payload as part of an Infor-
mational Exchange.



4.8 Informational Exchange


The Informational Exchange is designed as a one-way transmittal of infor-
mation that can be used for security association management.  The follow-
ing diagram shows the messages with possible payloads sent in each message
and notes for an example of the Informational Exchange.


                          INFORMATIONAL EXCHANGE

  __#___Initiator__Direction_Responder_______________NOTE_______________
   (1)  HDR*; N/D     =>                Error Notification or Deletion


In the first message (1), the initiator or responder transmits an ISAKMP
Notify or Delete payload.

If the Informational Exchange occurs prior to the exchange of keying me-
terial during an ISAKMP Phase 1 negotiation, there will be no protection
provided for the Informational Exchange.  Once keying material has been
exchanged or an ISAKMP SA has been established, the Informational Exchange
MUST be transmitted under the protection provided by the keying material
or the ISAKMP SA.

All exchanges are similar in that with the beginning of any exchange cryp-
tographic synchronization MUST occur.  The Informational Exchange is an
exchange and not an ISAKMP message.  Thus, the generation of an Initial-
ization Vector (IV) for an Informational Exchange SHOULD be independent
of IVs of other on-going communication.  This will ensure cryptographic
synchronization is maintained for existing communications and the Informa-
tional Exchange will be processed correctly.


5 ISAKMP Payload Processing


Section 3 describes the ISAKMP payloads.  These payloads are used in the
exchanges described in section 4 and can be used in exchanges defined for
a specific DOI. This section describes the processing for each of the
payloads.  This section suggests the logging of events to a system au-
dit file.  This action is controlled by a system security policy and is,


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therefore, only a suggested action.



5.1 General Message Processing


Every ISAKMP message has basic processing applied to insure protocol re-
liability, and to minimize threats, such as denial of service and replay
attacks.  All processing SHOULD include packet length checks to insure
the packet received is at least as long as the length given in the ISAKMP
Header.

When transmitting an ISAKMP message, the transmitting entity (initiator or
responder) MUST do the following:


1.  Set a timer and initialize a retry counter.

2.  If the timer expires, the ISAKMP message is resent and the retry
    counter is decremented.

3.  If the retry counter reaches zero (0), the event, RETRY LIMIT
    REACHED, MAY be logged in the appropriate system audit file.

4.  The ISAKMP protocol machine clears all states and returns to IDLE.


5.2 ISAKMP Header Processing


When creating an ISAKMP message, the transmitting entity (initiator or
responder) MUST do the following:


1.  Create the respective cookie.  See section 2.5.3 for details.

2.  Determine the relevant security characteristics of the session (i.e.
    DOI and situation).

3.  Construct an ISAKMP Header with fields as described in section 3.1.

4.  Construct other ISAKMP payloads, depending on the exchange type.

5.  Transmit the message to the destination host as described in section
    5.1.


When an ISAKMP message is received, the receiving entity (initiator or
responder) MUST do the following:



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1.  Verify the Initiator and Responder ``cookies''.  If the cookie
    validation fails, the message is discarded and the following actions
    are taken:


   (a)  The event, INVALID COOKIE, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-COOKIE message type MAY be sent to the transmitting
        entity.  This action is dictated by a system security policy.


2.  Check the Next Payload field to confirm it is valid.  If the Next
    Payload field validation fails, the message is discarded and the
    following actions are taken:


   (a)  The event, INVALID NEXT PAYLOAD, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-PAYLOAD-TYPE message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


3.  Check the Major and Minor Version fields to confirm they are correct.
    If the Version field validation fails, the message is discarded and
    the following actions are taken:


   (a)  The event, INVALID ISAKMP VERSION, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-MAJOR-VERSION or INVALID-MINOR-VERSION message type
        MAY be sent to the transmitting entity.  This action is dictated
        by a system security policy.


4.  Check the Exchange Type field to confirm it is valid.  If the
    Exchange Type field validation fails, the message is discarded and
    the following actions are taken:


   (a)  The event, INVALID EXCHANGE TYPE, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-EXCHANGE-TYPE message type MAY be sent to the


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        transmitting entity.  This action is dictated by a system
        security policy.


5.  Check the Flags field to ensure it contains correct values.  If the
    Flags field validation fails, the message is discarded and the
    following actions are taken:


   (a)  The event, INVALID FLAGS, MAY be logged in the appropriate system
        audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-FLAGS message type MAY be sent to the transmitting
        entity.  This action is dictated by a system security policy.


6.  Check the Message ID field to ensure it contains correct values.  If
    the Message ID validation fails, the message is discarded and the
    following actions are taken:


   (a)  The event, INVALID MESSAGE ID, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-MESSAGE-ID message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


7.  Processing of the ISAKMP message continues using the value in the
    Next Payload field.



5.3 Generic Payload Header Processing


When creating any of the ISAKMP Payloads described in sections 3.4 through
3.15 a Generic Payload Header is placed at the beginning of these pay-
loads.  When creating the Generic Payload Header, the transmitting entity
(initiator or responder) MUST do the following:


1.  Place the value of the Next Payload in the Next Payload field.  These
    values are described in section 3.1.

2.  Place the value zero (0) in the RESERVED field.

3.  Place the length (in octets) of the payload in the Payload Length
    field.

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4.  Construct the payloads as defined in the remainder of this section.



When any of the ISAKMP Payloads are received, the receiving entity (ini-
tiator or responder) MUST do the following:


1.  Check the Next Payload field to confirm it is valid.  If the Next
    Payload field validation fails, the message is discarded and the
    following actions are taken:


   (a)  The event, INVALID NEXT PAYLOAD, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-PAYLOAD-TYPE message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Verify the RESERVED field contains the value zero.  If the value in
    the RESERVED field is not zero, the message is discarded and the
    following actions are taken:


   (a)  The event, INVALID RESERVED FIELD, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the BAD-PROPOSAL-SYNTAX or PAYLOAD-MALFORMED message type MAY be
        sent to the transmitting entity.  This action is dictated by a
        system security policy.


3.  Process the remaining payloads as defined by the Next Payload field.


5.4 Security Association Payload Processing


When creating a Security Association Payload, the transmitting entity
(initiator or responder) MUST do the following:


1.  Determine the Domain of Interpretation for which this negotiation is
    being performed.

2.  Determine the situation within the determined DOI for which this
    negotiation is being performed.


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3.  Determine the proposal(s) and transform(s) within the situation.
    These are described, respectively, in sections 3.5 and 3.6.

4.  Construct a Security Association payload.

5.  Transmit the message to the receiving entity as described in section
    5.1.



When a Security Association payload is received, the receiving entity
(initiator or responder) MUST do the following:


1.  Determine if the Domain of Interpretation (DOI) is supported.  If the
    DOI determination fails, the message is discarded and the following
    actions are taken:


   (a)  The event, INVALID DOI, MAY be logged in the appropriate system
        audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the DOI-NOT-SUPPORTED message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Determine if the given situation can be protected.  If the Situation
    determination fails, the message is discarded and the following
    actions are taken:


   (a)  The event, INVALID SITUATION, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the SITUATION-NOT-SUPPORTED message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


3.  Process the remaining payloads (i.e.  Proposal, Transform) of the
    Security Association Payload.  If the Security Association Proposal
    (as described in sections 5.5 and 5.6) is not accepted, then the
    following actions are taken:


   (a)  The event, INVALID PROPOSAL, MAY be logged in the appropriate
        system audit file.



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   (b)  An Informational Exchange with a Notification payload containing
        the NO-PROPOSAL-CHOSEN message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



5.5 Proposal Payload Processing


When creating a Proposal Payload, the transmitting entity (initiator or
responder) MUST do the following:


1.  Determine the Protocol for this proposal.

2.  Determine the number of proposals to be offered for this protocol and
    the number of transforms for each proposal.  Transforms are described
    in section 3.6.

3.  Generate a unique pseudo-random SPI.

4.  Construct a Proposal payload.


When a Proposal payload is received, the receiving entity (initiator or
responder) MUST do the following:


1.  Determine if the Protocol is supported.  If the Protocol-ID field is
    invalid, the payload is discarded and the following actions are
    taken:


   (a)  The event, INVALID PROTOCOL, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-PROTOCOL-ID message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Determine if the SPI is valid.  If the SPI is invalid, the payload is
    discarded and the following actions are taken:


   (a)  The event, INVALID SPI, MAY be logged in the appropriate system
        audit file.




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   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-SPI message type MAY be sent to the transmitting
        entity.  This action is dictated by a system security policy.


3.  Ensure the Proposals are presented according to the details given in
    section 3.5 and 4.2.  If the proposals are not formed correctly, the
    following actions are taken:


   (a)  Possible events, BAD PROPOSAL SYNTAX, INVALID PROPOSAL, are
        logged in the appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the BAD-PROPOSAL-SYNTAX or PAYLOAD-MALFORMED message type MAY be
        sent to the transmitting entity.  This action is dictated by a
        system security policy.


4.  Process the Proposal and Transform payloads as defined by the Next
    Payload field.  Examples of processing these payloads are given in
    section 4.2.1.



5.6 Transform Payload Processing


When creating a Transform Payload, the transmitting entity (initiator or
responder) MUST do the following:


1.  Determine the Transform # for this transform.

2.  Determine the number of transforms to be offered for this proposal.
    Transforms are described in sections 3.6.

3.  Construct a Transform payload.


When a Transform payload is received, the receiving entity (initiator or
responder) MUST do the following:


1.  Determine if the Transform is supported.  If the Transform-ID field
    contains an unknown or unsupported value, then that Transform payload
    MUST be ignored and MUST NOT cause the generation of an INVALID
    TRANSFORM event.  If the Transform-ID field is invalid, the payload
    is discarded and the following actions are taken:




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   (a)  The event, INVALID TRANSFORM, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-TRANSFORM-ID message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Ensure the Transforms are presented according to the details given in
    section 3.6 and 4.2.  If the transforms are not formed correctly, the
    following actions are taken:


   (a)  Possible events, BAD PROPOSAL SYNTAX, INVALID TRANSFORM, INVALID
        ATTRIBUTES, are logged in the appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the BAD-PROPOSAL-SYNTAX, PAYLOAD-MALFORMED or ATTRIBUTES-NOT-
        SUPPORTED message type MAY be sent to the transmitting entity.
        This action is dictated by a system security policy.


3.  Process the subsequent Transform and Proposal payloads as defined by
    the Next Payload field.  Examples of processing these payloads are
    given in section 4.2.1.



5.7 Key Exchange Payload Processing


When creating a Key Exchange Payload, the transmitting entity (initiator
or responder) MUST do the following:


1.  Determine the Key Exchange to be used as defined by the DOI.

2.  Determine the usage of the Key Exchange Data field as defined by the
    DOI.

3.  Construct a Key Exchange payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When a Key Exchange payload is received, the receiving entity (initiator
or responder) MUST do the following:




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1.  Determine if the Key Exchange is supported.  If the Key Exchange
    determination fails, the message is discarded and the following
    actions are taken:


   (a)  The event, INVALID KEY INFORMATION, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-KEY-INFORMATION message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



5.8 Identification Payload Processing


When creating an Identification Payload, the transmitting entity (initia-
tor or responder) MUST do the following:


1.  Determine the Identification information to be used as defined by the
    DOI (and possibly the situation).

2.  Determine the usage of the Identification Data field as defined by
    the DOI.

3.  Construct an Identification payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When an Identification payload is received, the receiving entity (initia-
tor or responder) MUST do the following:


1.  Determine if the Identification Type is supported.  This may be based
    on the DOI and Situation.  If the Identification determination fails,
    the message is discarded and the following actions are taken:


   (a)  The event, INVALID ID INFORMATION, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-ID-INFORMATION message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



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5.9 Certificate Payload Processing


When creating a Certificate Payload, the transmitting entity (initiator or
responder) MUST do the following:



1.  Determine the Certificate Encoding to be used.  This may be specified
    by the DOI.

2.  Ensure the existence of a certificate formatted as defined by the
    Certificate Encoding.

3.  Construct a Certificate payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When a Certificate payload is received, the receiving entity (initiator or
responder) MUST do the following:


1.  Determine if the Certificate Encoding is supported.  If the
    Certificate Encoding is not supported, the payload is discarded and
    the following actions are taken:


   (a)  The event, INVALID CERTIFICATE TYPE, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-CERT-ENCODING message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Process the Certificate Data field.  If the Certificate Data is
    invalid or improperly formatted, the payload is discarded and the
    following actions are taken:


   (a)  The event, INVALID CERTIFICATE, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-CERTIFICATE message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



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5.10 Certificate Request Payload Processing


When creating a Certificate Request Payload, the transmitting entity (ini-
tiator or responder) MUST do the following:



1.  Determine the type of Certificate Encoding to be requested.  This may
    be specified by the DOI.

2.  Determine the name of an acceptable Certificate Authority which is to
    be requested (if applicable).

3.  Construct a Certificate Request payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When a Certificate Request payload is received, the receiving entity (ini-
tiator or responder) MUST do the following:


1.  Determine if the Certificate Encoding is supported.  If the
    Certificate Encoding is invalid, the payload is discarded and the
    following actions are taken:


   (a)  The event, INVALID CERTIFICATE TYPE, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-CERT-ENCODING message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


    If the Certificate Encoding is not supported, the payload is
    discarded and the following actions are taken:


   (a)  The event, CERTIFICATE TYPE UNSUPPORTED, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the CERT-TYPE-UNSUPPORTED message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.




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2.  Determine if the Certificate Authority is supported for the specified
    Certificate Encoding.  If the Certificate Authority is invalid or
    improperly formatted, the payload is discarded and the following
    actions are taken:


   (a)  The event, INVALID CERTIFICATE AUTHORITY, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-CERT-AUTHORITY message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


3.  Process the Certificate Request.  If a requested Certificate Type
    with the specified Certificate Authority is not available, then the
    payload is discarded and the following actions are taken:


   (a)  The event, CERTIFICATE-UNAVAILABLE, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the CERTIFICATE-UNAVAILABLE message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



5.11 Hash Payload Processing


When creating a Hash Payload, the transmitting entity (initiator or re-
sponder) MUST do the following:


1.  Determine the Hash function to be used as defined by the SA
    negotiation.

2.  Determine the usage of the Hash Data field as defined by the DOI.

3.  Construct a Hash payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When a Hash payload is received, the receiving entity (initiator or re-
sponder) MUST do the following:



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1.  Determine if the Hash is supported.  If the Hash determination fails,
    the message is discarded and the following actions are taken:


   (a)  The event, INVALID HASH INFORMATION, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-HASH-INFORMATION message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Perform the Hash function as outlined in the DOI and/or Key Exchange
    protocol documents.  If the Hash function fails, the message is
    discarded and the following actions are taken:


   (a)  The event, INVALID HASH VALUE, MAY be logged in the appropriate
        system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the AUTHENTICATION-FAILED message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



5.12 Signature Payload Processing


When creating a Signature Payload, the transmitting entity (initiator or
responder) MUST do the following:


1.  Determine the Signature function to be used as defined by the SA
    negotiation.

2.  Determine the usage of the Signature Data field as defined by the
    DOI.

3.  Construct a Signature payload.

4.  Transmit the message to the receiving entity as described in section
    5.1.


When a Signature payload is received, the receiving entity (initiator or
responder) MUST do the following:




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1.  Determine if the Signature is supported.  If the Signature
    determination fails, the message is discarded and the following
    actions are taken:


   (a)  The event, INVALID SIGNATURE INFORMATION, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the INVALID-SIGNATURE message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.


2.  Perform the Signature function as outlined in the DOI and/or Key
    Exchange protocol documents.  If the Signature function fails, the
    message is discarded and the following actions are taken:


   (a)  The event, INVALID SIGNATURE VALUE, MAY be logged in the
        appropriate system audit file.

   (b)  An Informational Exchange with a Notification payload containing
        the AUTHENTICATION-FAILED message type MAY be sent to the
        transmitting entity.  This action is dictated by a system
        security policy.



5.13 Nonce Payload Processing


When creating a Nonce Payload, the transmitting entity (initiator or re-
sponder) MUST do the following:


1.  Create a unique random value to be used as a nonce.

2.  Construct a Nonce payload.

3.  Transmit the message to the receiving entity as described in section
    5.1.


When a Nonce payload is received, the receiving entity (initiator or re-
sponder) MUST do the following:


1.  There are no specific procedures for handling Nonce payloads.  The
    procedures are defined by the exchange types (and possibly the DOI
    and Key Exchange descriptions).


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5.14 Notification Payload Processing


During communications it is possible that errors may occur.  The Infor-
mational Exchange with a Notify Payload provides a controlled method of
informing a peer entity that errors have occurred during protocol process-
ing.  It is RECOMMENDED that Notify Payloads be sent in a separate Infor-
mational Exchange rather than appending a Notify Payload to an existing
exchange.

When creating a Notification Payload, the transmitting entity (initiator
or responder) MUST do the following:



1.  Determine the DOI for this Notification.

2.  Determine the Protocol-ID for this Notification.

3.  Determine the SPI size based on the Protocol-ID field.  This field is
    necessary because different security protocols have different SPI
    sizes.  For example, ISAKMP combines the Initiator and Responder
    cookie pair (16 octets) as a SPI, while ESP and AH have 8 octet SPIs.

4.  Determine the Notify Message Type based on the error or status
    message desired.

5.  Determine the SPI which is associated with this notification.

6.  Determine if additional Notification Data is to be included.  This is
    additional information specified by the DOI.

7.  Construct a Notification payload.

8.  Transmit the message to the receiving entity as described in section
    5.1.


Because the Informational Exchange with a Notification payload is a uni-
directional message a retransmission will not be performed.  The local
security policy will dictate the procedures for continuing.  However, we
RECOMMEND that a NOTIFICATION PAYLOAD ERROR event be logged in the appro-
priate system audit file by the receiving entity.

If the Informational Exchange occurs prior to the exchange of keying ma-
terial during an ISAKMP Phase 1 negotiation there will be no protection
provided for the Informational Exchange.  Once the keying material has
been exchanged or the ISAKMP SA has been established, the Informational
Exchange MUST be transmitted under the protection provided by the keying
material or the ISAKMP SA.

When a Notification payload is received, the receiving entity (initiator

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or responder) MUST do the following:



1.  Determine if the Informational Exchange has any protection applied to
    it by checking the Encryption Bit and the Authentication Only Bit in
    the ISAKMP Header.  If the Encryption Bit is set, i.e.  the Informa-
    tional Exchange is encrypted, then the message MUST be decrypted
    using the (in-progress or completed) ISAKMP SA. Once the decryption
    is complete the processing can continue as described below.  If the
    Authentication Only Bit is set, then the message MUST be authenti-
    cated using the (in-progress or completed) ISAKMP SA. Once the
    authentication is completed, the processing can continue as described
    below.  If the Informational Exchange is not encrypted or authentica-
    tion, the payload processing can continue as described below.

2.  Determine if the Domain of Interpretation (DOI) is supported.  If the
    DOI determination fails, the payload is discarded and the following
    action is taken:


   (a)  The event, INVALID DOI, MAY be logged in the appropriate system
        audit file.


3.  Determine if the Protocol-Id is supported.  If the Protocol-Id
    determination fails, the payload is discarded and the following
    action is taken:


   (a)  The event, INVALID PROTOCOL-ID, MAY be logged in the appropriate
        system audit file.


4.  Determine if the SPI is valid.  If the SPI is invalid, the payload is
    discarded and the following action is taken:


   (a)  The event, INVALID SPI, MAY be logged in the appropriate system
        audit file.


5.  Determine if the Notify Message Type is valid.  If the Notify Message
    Type is invalid, the payload is discarded and the following action is
    taken:


   (a)  The event, INVALID MESSAGE TYPE, MAY be logged in the appropriate
        system audit file.


6.  Process the Notification payload, including additional Notification

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    Data, and take appropriate action, according to local security
    policy.



5.15 Delete Payload Processing


During communications it is possible that hosts may be compromised or that
information may be intercepted during transmission.  Determining whether
this has occurred is not an easy task and is outside the scope of this
Internet-Draft.  However, if it is discovered that transmissions are being
compromised, then it is necessary to establish a new SA and delete the
current SA.

The Informational Exchange with a Delete Payload provides a controlled
method of informing a peer entity that the transmitting entity has deleted
the SA(s).  Deletion of Security Associations MUST always be performed un-
der the protection of an ISAKMP SA. The receiving entity SHOULD clean up
its local SA database.  However, upon receipt of a Delete message the SAs
listed in the Security Parameter Index (SPI) field of the Delete payload
cannot be used with the transmitting entity.  The SA Establishment proce-
dure must be invoked to re-establish secure communications.

When creating a Delete Payload, the transmitting entity (initiator or re-
sponder) MUST do the following:


1.  Determine the DOI for this Deletion.

2.  Determine the Protocol-ID for this Deletion.

3.  Determine the SPI size based on the Protocol-ID field.  This field is
    necessary because different security protocols have different SPI
    sizes.  For example, ISAKMP combines the Initiator and Responder
    cookie pair (16 octets) as a SPI, while ESP and AH have 8 octet SPIs.

4.  Determine the # of SPIs to be deleted for this protocol.

5.  Determine the SPI(s) which is (are) associated with this deletion.

6.  Construct a Delete payload.

7.  Transmit the message to the receiving entity as described in section
    5.1.


Because the Informational Exchange with a Delete payload is a unidirec-
tional message a retransmission will not be performed.  The local security
policy will dictate the procedures for continuing.  However, we RECOMMEND
that a DELETE PAYLOAD ERROR event be logged in the appropriate system au-


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dit file by the receiving entity.

As described above, the Informational Exchange with a Delete payload MUST
be transmitted under the protection provided by an ISAKMP SA.

When a Delete payload is received, the receiving entity (initiator or re-
sponder) MUST do the following:



1.  Because the Informational Exchange is protected by some security
    service (e.g.  authentication for an Auth-Only SA, encryption for
    other exchanges), the message MUST have these security services
    applied using the ISAKMP SA. Once the security service processing is
    complete the processing can continue as described below.  Any errors
    that occur during the security service processing will be evident
    when checking information in the Delete payload.  The local security
    policy SHOULD dictate any action to be taken as a result of security
    service processing errors.

2.  Determine if the Domain of Interpretation (DOI) is supported.  If the
    DOI determination fails, the payload is discarded and the following
    action is taken:


   (a)  The event, INVALID DOI, MAY be logged in the appropriate system
        audit file.


3.  Determine if the Protocol-Id is supported.  If the Protocol-Id
    determination fails, the payload is discarded and the following
    action is taken:


   (a)  The event, INVALID PROTOCOL-ID, MAY be logged in the appropriate
        system audit file.


4.  Determine if the SPI is valid for each SPI included in the Delete
    payload.  For each SPI that is invalid, the following action is
    taken:


   (a)  The event, INVALID SPI, MAY be logged in the appropriate system
        audit file.


5.  Process the Delete payload and take appropriate action, according to
    local security policy.  As described above, one appropriate action
    SHOULD include cleaning up the local SA database.



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6 Conclusions


The Internet Security Association and Key Management Protocol (ISAKMP) is
a well designed protocol aimed at the Internet of the future.  The mas-
sive growth of the Internet will lead to great diversity in network uti-
lization, communications, security requirements, and security mechanisms.
ISAKMP contains all the features that will be needed for this dynamic and
expanding communications environment.

ISAKMP's Security Association (SA) feature coupled with authentication
and key establishment provides the security and flexibility that will be
needed for future growth and diversity.  This security diversity of multi-
ple key exchange techniques, encryption algorithms, authentication mecha-
nisms, security services, and security attributes will allow users to se-
lect the appropriate security for their network, communications, and secu-
rity needs.  The SA feature allows users to specify and negotiate security
requirements with other users.  An additional benefit of supporting multi-
ple techniques in a single protocol is that as new techniques are devel-
oped they can easily be added to the protocol.  This provides a path for
the growth of Internet security services.  ISAKMP supports both publicly
or privately defined SAs, making it ideal for government, commercial, and
private communications.

ISAKMP provides the ability to establish SAs for multiple security proto-
cols and applications.  These protocols and applications may be session-
oriented or sessionless.  Having one SA establishment protocol that sup-
ports multiple security protocols eliminates the need for multiple, nearly
identical authentication, key exchange and SA establishment protocols when
more than one security protocol is in use or desired.  Just as IP has pro-
vided the common networking layer for the Internet, a common security es-
tablishment protocol is needed if security is to become a reality on the
Internet.  ISAKMP provides the common base that allows all other security
protocols to interoperate.

ISAKMP follows good security design principles.  It is not coupled to
other insecure transport protocols, therefore it is not vulnerable or
weakened by attacks on other protocols.  Also, when more secure transport
protocols are developed, ISAKMP can be easily migrated to them.  ISAKMP
also provides protection against protocol related attacks.  This protec-
tion provides the assurance that the SAs and keys established are with the
desired party and not with an attacker.

ISAKMP also follows good protocol design principles.  Protocol specific
information only is in the protocol header, following the design prin-
ciples of IPv6.  The data transported by the protocol is separated into
functional payloads.  As the Internet grows and evolves, new payloads to
support new security functionality can be added without modifying the en-
tire protocol.




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A ISAKMP Security Association Attributes



A.1 Background/Rationale


As detailed in previous sections, ISAKMP is designed to provide a flexible
and extensible framework for establishing and managing Security Associa-
tions and cryptographic keys.  The framework provided by ISAKMP consists
of header and payload definitions, exchange types for guiding message and
payload exchanges, and general processing guidelines.  ISAKMP does not
define the mechanisms that will be used to establish and manage Security
Associations and cryptographic keys in an authenticated and confidential
manner.  The definition of mechanisms and their application is the purview
of individual Domains of Interpretation (DOIs).

This section describes the ISAKMP values for the Internet IP Security DOI,
supported security protocols, and identification values for ISAKMP Phase 1
negotiations.  The Internet IP Security DOI is MANDATORY to implement for
IP Security.  [Oakley] and [IKE] describe, in detail, the mechanisms and
their application for establishing and managing Security Associations and
cryptographic keys for IP Security.


A.2 Internet IP Security DOI Assigned Value


As described in [IPDOI], the Internet IP Security DOI Assigned Number is
one (1).


A.3 Supported Security Protocols


Values for supported security protocols are specified in the most recent
``Assigned Numbers'' RFC [STD-2].  Presented in the following table are
the values for the security protocols supported by ISAKMP for the Internet
IP Security DOI.


                        _Protocol_Assigned_Value__
                         RESERVED        0
                         ISAKMP          1


All DOIs MUST reserve ISAKMP with a Protocol-ID of 1.  All other security
protocols within that DOI will be numbered accordingly.

Security protocol values 2-15359 are reserved to IANA for future use.
Values 15360-16383 are permanently reserved for private use amongst mu-


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tually consenting implementations.  Such private use values are unlikely
to be interoperable across different implementations.



A.4 ISAKMP Identification Type Values


The following table lists the assigned values for the Identification Type
field found in the Identification payload during a generic Phase 1 ex-
change, which is not for a specific protocol.


                        ______ID_Type_______Value_
                        ID_IPV4_ADDR          0
                        ID_IPV4_ADDR_SUBNET   1
                        ID_IPV6_ADDR          2
                        ID_IPV6_ADDR_SUBNET   3



A.4.1 ID_IPV4_ADDR


The ID_IPV4_ADDR type specifies a single four (4) octet IPv4 address.


A.4.2 ID_IPV4_ADDR_SUBNET


The ID_IPV4_ADDR_SUBNET type specifies a range of IPv4 addresses, repre-
sented by two four (4) octet values.  The first value is an IPv4 address.
The second is an IPv4 network mask.  Note that ones (1s) in the network
mask indicate that the corresponding bit in the address is fixed, while
zeros (0s) indicate a "wildcard" bit.


A.4.3 ID_IPV6_ADDR


The ID_IPV6_ADDR type specifies a single sixteen (16) octet IPv6 address.


A.4.4 ID_IPV6_ADDR_SUBNET


The ID_IPV6_ADDR_SUBNET type specifies a range of IPv6 addresses, repre-
sented by two sixteen (16) octet values.  The first value is an IPv6 ad-
dress.  The second is an IPv6 network mask.  Note that ones (1s) in the
network mask indicate that the corresponding bit in the address is fixed,
while zeros (0s) indicate a "wildcard" bit.


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B Defining a new Domain of Interpretation


The Internet DOI may be sufficient to meet the security requirements of
a large portion of the internet community.  However, some groups may have
a need to customize some aspect of a DOI, perhaps to add a different set
of cryptographic algorithms, or perhaps because they want to make their
security-relevant decisions based on something other than a host id or
user id.  Also, a particular group may have a need for a new exchange
type, for example to support key management for multicast groups.

This section discusses guidelines for defining a new DOI. The full speci-
fication for the Internet DOI can be found in [IPDOI].

Defining a new DOI is likely to be a time-consuming process.  If at all
possible, it is recommended that the designer begin with an existing DOI
and customize only the parts that are unacceptable.

If a designer chooses to start from scratch, the following MUST be de-
fined:



 o  A ``situation'':  the set of information that will be used to
    determine the required security services.

 o  The set of security policies that must be supported.

 o  A scheme for naming security-relevant information, including
    encryption algorithms, key exchange algorithms, etc.

 o  A syntax for the specification of proposed security services,
    attributes, and certificate authorities.

 o  The specific formats of the various payload contents.

 o  Additional exchange types, if required.


B.1 Situation


The situation is the basis for deciding how to protect a communications
channel.  It must contain all of the data that will be used to determine
the types and strengths of protections applied in an SA. For example, a
US Department of Defense DOI would probably use unpublished algorithms
and have additional special attributes to negotiate.  These additional
security attributes would be included in the situation.





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B.2 Security Policies


Security policies define how various types of information must be cate-
gorized and protected.  The DOI must define the set of security policies
supported, because both parties in a negotiation must trust that the other
party understands a situation, and will protect information appropriately,
both in transit and in storage.  In a corporate setting, for example, both
parties in a negotiation must agree to the meaning of the term ``propri-
etary information'' before they can negotiate how to protect it.

Note that including the required security policies in the DOI only speci-
fies that the participating hosts understand and implement those policies
in a full system context.



B.3 Naming Schemes


Any DOI must define a consistent way to name cryptographic algorithms,
certificate authorities, etc.  This can usually be done by using IANA nam-
ing conventions, perhaps with some private extensions.


B.4 Syntax for Specifying Security Services


In addition to simply specifying how to name entities, the DOI must also
specify the format for complete proposals of how to protect traffic under
a given situation.


B.5 Payload Specification


The DOI must specify the format of each of the payload types.  For several
of the payload types, ISAKMP has included fields that would have to be
present across all DOI (such as a certificate authority in the certificate
payload, or a key exchange identifier in the key exchange payload).


B.6 Defining new Exchange Types


If the basic exchange types are inadequate to meet the requirements within
a DOI, a designer can define up to thirteen extra exchange types per DOI.
The designer creates a new exchange type by choosing an unused exchange
type value, and defining a sequence of messages composed of strings of the
ISAKMP payload types.



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Note that any new exchange types must be rigorously analyzed for vulner-
abilities.  Since this is an expensive and imprecise undertaking, a new
exchange type should only be created when absolutely necessary.


















































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Security Considerations


Cryptographic analysis techniques are improving at a steady pace.  The
continuing improvement in processing power makes once computationally pro-
hibitive cryptographic attacks more realistic.  New cryptographic algo-
rithms and public key generation techniques are also being developed at a
steady pace.  New security services and mechanisms are being developed at
an accelerated pace.  A consistent method of choosing from a variety of
security services and mechanisms and to exchange attributes required by
the mechanisms is important to security in the complex structure of the
Internet.  However, a system that locks itself into a single cryptographic
algorithm, key exchange technique, or security mechanism will become in-
creasingly vulnerable as time passes.

UDP is an unreliable datagram protocol and therefore its use in ISAKMP in-
troduces a number of security considerations.  Since UDP is unreliable,
but a key management protocol must be reliable, the reliability is built
into ISAKMP. While ISAKMP utilizes UDP as its transport mechanism, it
doesn't rely on any UDP information (e.g.  checksum, length) for its pro-
cessing.

Another issue that must be considered in the development of ISAKMP is the
effect of firewalls on the protocol.  Many firewalls filter out all UDP
packets, making reliance on UDP questionable in certain environments.

A number of very important security considerations are presented in
[RFC-1825].  One bears repeating.  Once a private session key is created,
it must be safely stored.  Failure to properly protect the private key
from access both internal and external to the system completely nullifies
any protection provided by the IP Security services.



IANA Considerations


This document contains many "magic" numbers to be maintained by the IANA.
This section explains the criteria to be used by the IANA to assign addi-
tional numbers in each of these lists.


Domain of Interpretation


The Domain of Interpretation (DOI) is a 32-bit field which identifies the
domain under which the security association negotiation is taking place.
Requests for assignments of new DOIs must be accompanied by a standards-
track RFC which describes the specific domain.




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Supported Security Protocols


ISAKMP is designed to provide security association negotiation and key
management for many security protocols.  Requests for identifiers for ad-
ditional security protocols must be accompanied by a standards-track RFC
which describes the security protocol and its relationship to ISAKMP.



Acknowledgements


Dan Harkins, Dave Carrel, and Derrell Piper of Cisco Systems provided
design assistance with the protocol and coordination for the [IKE] and
[IPDOI] documents.

Hilarie Orman, via the Oakley key exchange protocol, has significantly
influenced the design of ISAKMP.

Marsha Gross, Bill Kutz, Mike Oehler, Pete Sell, and Ruth Taylor provided
significant input and review to this document.

Scott Carlson ported the TIS DNSSEC prototype to FreeBSD for use with the
ISAKMP prototype.

Jeff Turner and Steve Smalley contributed to the prototype development and
integration with ESP and AH.

Mike Oehler and Pete Sell performed interoperability testing with other
ISAKMP implementors.

Thanks to Carl Muckenhirn of SPARTA, Inc.  for his assistance with LaTeX.




















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References


[ANSI] ANSI, X9.42:  Public Key Cryptography for the Financial Services
     Industry -- Establishment of Symmetric Algorithm Keys Using
     Diffie-Hellman, Working Draft, April 19, 1996.

[BC] Ballardie, A. and J. Crowcroft, Multicast-specific Security Threats
     and Countermeasures, Proceedings of 1995 ISOC Symposium on Networks
     & Distributed Systems Security, pp. 17-30, Internet Society, San
     Diego, CA, February 1995.

[Berge] Berge, N.H., UNINETT PCA Policy Statements, Internet-Draft, work
     in progress, November, 1995.

[CW87] Clark, D.D. and D.R. Wilson, A Comparison of Commercial and
     Military Computer Security Policies, Proceedings of the IEEE
     Symposium on Security & Privacy, Oakland, CA, 1987, pp. 184-193.

[DNSSEC] D. Eastlake III, Domain Name System Protocol Security
     Extensions, Internet-Draft:  draft-ietf-dnssec-secext2-03.txt, Work
     in Progress, January 1998.

[DOW92] Diffie, W., M.Wiener, P. Van Oorschot, Authentication and
     Authenticated Key Exchanges, Designs, Codes, and Cryptography, 2,
     107-125, Kluwer Academic Publishers, 1992.

[IAB] Bellovin, S., Report of the IAB Security Architecture Workshop,
     Internet-Draft:  draft-iab-secwks-report-00.txt, Work in Progress,
     November 1997.

[IKE] Harkins, D. and D. Carrel, The Internet Key Exchange (IKE),
     Internet-Draft:  draft-ietf-ipsec-isakmp-oakley-06.txt, Work in
     Progress, February 1998.

[IPDOI] Derrell Piper, The Internet IP Security Domain of Interpretation
     for ISAKMP, Internet-Draft:  draft-ietf-ipsec-ipsec-doi-07.txt, Work
     in Progress, February 1998.

[Karn] Karn, P. and B. Simpson, Photuris:  Session Key Management
     Protocol, Internet-Draft:  draft-simpson-photuris-15.txt, Work in
     Progress, July 1997.

[Kent94] Steve Kent, IPSEC SMIB, e-mail to ipsec@ans.net, August 10,
     1994.

[Oakley] H. K. Orman, The Oakley Key Determination Protocol, Internet-
     Draft:  draft-ietf-ipsec-oakley-02.txt, Work in Progress, July 1997.

[RFC-1422] Steve Kent, Privacy Enhancement for Internet Electronic Mail:
     Part II: Certificate-Based Key Management, RFC-1422, February 1993.


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[RFC-1825] Randall Atkinson, Security Architecture for the Internet
     Protocol, RFC-1825, August, 1995.

[RFC-1949] A. Ballardie, Scalable Multicast Key Distribution, RFC-1949,
     May 1996.

[RFC-2093] Harney, H. and C. Muckenhirn, Group Key Management Protocol
     (GKMP) Specification, SPARTA, Inc., RFC-2093, July 1997.

[RFC-2094] Harney, H. and C. Muckenhirn, Group Key Management Protocol
     (GKMP) Architecture, SPARTA, Inc., RFC-2094, July 1997.

[RFC-2119] S. Bradner, Key Words for use in RFCs to Indicate Requirement
     Levels, Harvard University, RFC-2119, March 1997.

[Schneier] Bruce Schneier, Applied Cryptography - Protocols, Algorithms,
     and Source Code in C (Second Edition), John Wiley & Sons, Inc.,
     1996.

[STD-2] Reynolds, J. and J. Postel, Assigned Numbers, STD 2, October,
     1994.
































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Addresses of Authors

The authors can be contacted at:

     Douglas Maughan
         Phone:  301-688-0847
         E-mail:wdm@tycho.ncsc.mil

     Mark Schneider
         Phone:  301-688-0851
         E-mail:mss@tycho.ncsc.mil

         National Security Agency
         ATTN: R23
         9800 Savage Road
         Ft.  Meade, MD. 20755-6000

     Mark Schertler
         Terisa Systems, Inc.
         4984 El Camino Real
         Los Altos, CA. 94022
         Phone:  650-919-1773
         E-mail:mjs@terisa.com

     Jeff Turner
         RABA Technologies, Inc.
         10500 Little Patuxent Parkway
         Columbia, MD. 21044
         Phone:  410-715-9399
         E-mail:jeff.turner@raba.com























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